Abstract: The Shishugou Formation of the northeastern Junggar Basin, Xinjiang Uygur Autonomous Region, China was deposited during the late Middle Jurassic through early Late Jurassic(Eberth et al.,2001). The giant sauropod Mamenchisaurus sinocanadorum(Russell and Zheng,1993) is known from the formation, but the largest theropod known is the relatively small Sinraptor dongi (Currie and Zhao,1993), with an estimated body length of 7.2 meters. Our expeditions from 2001 through 2007 in this formation resulted in the discoveries of many articulated skeletons and many isolated elements as well(Clark et al.,2006). Here we report an isolated large theropod tooth collected from the upper part of the formation at the Wucaiwan locality in the Junggar Basin. This isolated tooth was preserved within a partial sauropod skeleton, which might have been a prey of the bearer of the tooth. The dental description terms follow Smith et al.(2005). V 15310 is possibly a shed crown of a lateral tooth as a root is absent and a depression appears to be present at the base of the crown which might indicate the presence of a resorption pit. Like a typical theropod tooth crown, it is strongly compressed mediolaterally, curved posteriorly, and finely denticulated along the mesial and distal carinae(Fig.1A) V15310 is large in size. The apical length(AL) and crown height(CH) of the preservedtooth crown are 102 mm and 92 mm, respectively; the crown base length(CBL) and crown base width(CBW) are 35 and 18 mm, respectively; the crown base ratio(CBR) and crown height ratio(CHR) are 0.51 and 2.63, respectively. The tooth crown is much thicker labiolingually close to the mesial margin than to the distal margin and basally than apically; the lingual surface is much more convex mesiodistally than the labial surface. Both the mesial and distal carinae are well developed(Fig.1B,C). The mesial carina is slightly displaced onto the lingual surface apically. Small denticles are present along both carinae. The denticulated carina is 83 mm long along the distal margin and appears to be slightly shorter(80 mm long) along the mesial margin. The denticles are variable in size both between and along the carinae. The denticles are robust, with the labiolingual width of their basal sec-tions much greater than either their length or height. The middle denticles are slightly longer than the apical ones (0.57 and 0.49 mm in basal length, respectively, along the distal carina), and the latter longer than the basal ones (0.44 mm). An individual denticle is small relative to the tooth size. DSI(Denticle Size Index: defined as a ratio of denticle basal length against CH times 1000) is used to describe the relative size of denticles. DSI for the middle denticles along the distal carina of V 15310 is about 6, comparable to that in a similar-sized Tyrannosaurus tooth (about 7). It is about 20 in a Sinraptor tooth of 35 mm in CH. Denticle size is negatively correlated with the dental crown size among theropods and larger teeth normally bear smaller denticles(Farlow et al.,1991). The denticles along the distal carina are slender in labial or lingual view, being much higher than long(the basal ones are nearly twice as highas long, the middle ones about one-fourth higher than long, and the apical ones intermediate in proportions). They are slightly curved toward the crown tip, the apical ones more so than the basal ones. The apical ones are, however, more symmetrical than the basal ones in having a bluntly rounded tip whereas in the latter the more pointed tips curve slightly toward the crown tip. The inter-denticle notch(or slit) is shallow between the apical denticles but deep between the basal denticles (one-third the denticle height). Blood grooves are restricted between the adjacent denticles, with little extension onto the crown surface. The denticles along the mesial carina are similar in most features to the ones along the distal carina;a few differences exist, however. They are proportionally shorter(about as high as long) and thus look more robust than the denticles along the distal carina. Also unlike the latter, which are chisel-like with the base smoothly continuing onto the crown surface, the mesial denticles are somewhat bulbous basally. Weak enamel wrinkles are present on both the labial and lingual surfaces. The wrinkle rows are close and parallel to the denticles, and they are more developed close to the mesial carina than the distal carina. The wrinkles are arcuate, each about 4 mm long, and sweep slightly down and away from the carinae. Wear is present in several forms. The enamel is worn away around the tip, producing a slightly round tip. The wear facet along the mesial carina is large, extendsfrom the tip a length of about 10 mm, and is relatively flat basally. Some denticles along both mesial and distal carinae are abraded. The apical denticles are more heavily abraded than the more basal ones andthe denticles along the mesial carina are abraded more than along the distal carina. Several dental features are useful for inferring the affinities of V 15310. Enamel winkles have been reported in Carcharodontosaurus (Sereno et al.,1996), Giganotosaurus, some large tyrannosauroid teeth(Holtz, 2004), and isolated teeth from the Late Cretaceous of Japan and Brazil, respectively (Chure et al.,1999). They are also present in Sinraptor, though extremely weakly developed. In Carcha-rodontosaurus, Gigantosaurus and Sinraptor enamel wrinkles are across the crown surface; in tyrannosauroids and the isolated teeth from Japan and Brazil, enamel wrinkles are restricted to the mesial and/or distal margins.V 15310 is similar to the latter group, though more weakly developed. The distal denticles are significantly higher than the mesial denticles in V15310,a feature known in some dromaeosaurids, basal tyrannosauroids, Sinraptor, and Yangchuanosaurus, but V 15310 is more similar to Sinraptor and Yangchuanosaurus in that the mesial denticles are about as long as the distal ones. It is likely that V 15310 is a gigantic sinraptorid, either ataxon assignable or closely related to Yangchuanosaurus magnus, the longest tooth of which is 75 mm long in AL(Dong et al.,1983) or an old individual of Sinraptor. If the latter is true, Sinraptor might have an indeterminate growth strategy because the much smaller Sinraptor holotype is an adult individual(Currie and Zhao,1993). The most conspicuous feature is the large size of V 15310. Among the known theropod taxa (Smith,2005; Smith et al.,2005), only Tyrannosaurus, Carcharodontosaurus, Acrocanthosaurus and probably Giganotosaurus have a maximum CBI subequal to or larger than that of V15310; Tyrannosaurus, Acrocanthosaurus, Suchomimus, Gorgosaurus and Daspletosaurus have a maximum CBW subequal to or larger than that of V 15310; Tyrannosaurus, Carcharodontosaurus and probably Gigantosaurus have a maximum CH subequal to or larger than that of V15310; and only Tyrannosaurus has a maximum AL subequal to or larger than that of V15310. V 15310 is the largest known theropod tooth reported to date from Jurassic sediments. The discovery of V 15310 from the Shishugou Formation suggests that theropods evolved gigantic size as early as the early Late Jurassic, possibly co-evolving with large herbivorous dinosaurs such as mamenchisaurs that are common in this formation.