Abstract: A new species of galeaspid agnathan, Macrothyraspis longilanceus, is described from the Early Devonian Posongchong Formation ( Upper Pragian) at Zhichang village, Gumu Town, Wenshan District, southeastem Yunnan, China. As a typical non-marine deposit, the Posongchong Formation in southeastern Yunnan yields abundant remains of early vascular plants, antiarchs, sarcopterygians and galeaspids , the latter including Kwangnanaspis subtriangularis ( Cao, 1979) , Cantarostrataspis gengi, Cumuaspis rostrata ( Wang and Wang , 1992), Macrothyraspis longicornis (Pan, 1992) , Wenshanaspis zhichangensis and Sanqiaspis rostrata ( Zhao et al. , 2002) .