Abstract: The morphological and struetural characteristics of developing cocyst satellite cyst of Plasmodium yoelii nigeriensis in Anopheles dirus had been observed with the transmission electron microscopy and oil emersion microscopy for lμm section stained with Giemsa's solution. The sporoblast formation began with the appearance of vacuoles just beneath the oocyst capsule this followed by the subdivision of the cytoplasm, and sporoblast for mation. The next stage was indicated by budding of sPoroblast wilh the development of sporozoite. The satellite cysts were always developing normally, The satellite cysts could form late stage of sporoblast in the 7 day-oocyst. Most of the satellite cysts developed more slowly and the number of sporozoite buis also were fewer than that of their attaching oocysts.

Key words: Plasmodium yoelii nigeriensis, oocyst, satellite oocyst