The fine structural aspects of Macrocametogenesis and process of fertilization in Tyzzeria parvula were studied in tissue from the small intestine of infected young domestic geese via electron microscopy.Multi-membranous vesicles were present in both the nucleoplasm and cytoplasm of young macrogametocytes.It was observed that electron translucent vacuoles appeared to be ejected from the surface of the macrogametocytes. The wall forming bodies of type 1 (WF1) came irto view before the wall forming bodies of type 2(WF2). The WF1 was osmi-ophilic, and was smaller than tne WF2.WF1 and WF2 participated respectively in formation of the outer and inner layer of the oocyst wall. Fertilization of a macrogamete by a micro-gamete occurred prior to oocyst wall formation.Both the nucleus and the flagella of the mi-crogamete penetrated into the cytoplasm of macrogamete, but only the nucleus fused with the nucleus of macrogamete.