Coccidian oocysts were proved to be found in 70 of 100 collected Pirenella conica snails, with a natural infection of 70%. It was observed that, Pfeifferinella sp. was transferred between hepatopancreas and small intestine of snail. The prepatent period of Pfeifferinella sp. infecting P. conica snails ranged from 14-18 days and the patent period was reached 50 days (P.I.). Merogony stages were the early stages observed in this study. These stages were observed in the hepatopancreas and in a large clear parasiteophorous vacuole (PV). In snails killed 4 days P.I. immature meronts were measured 12 х 10 μm containing 8 nuclei. Meanwhile, mature meronts with about 6 differentiated merozoites were detected as early as 6 days P.I., and measured 3.1х1.4μm. The earliest gametogonic stages were seen in the intestine of Pirenella conica snails
killed 12 days P.I. Microgamonts contained about 4 nuclei and measured 7.9х6.7μm. The macrogamonts measured 7.3х5.6μm. Macrogametes were characterized by the presence of the vaginal tube, this tube measured 4.3х1.1μm. Fertilization was occurred in the intestine of the infected snails at 12 days P.I. Zygotes developed into young oocysts after fertilization. Sporogony occurred in the intestine. In the earliest stage, the nucleus of young oocyst was occupied the central position that were observed through the examination of the intestine of infected Pirenella snails at 14 day P.I. These oocysts were found to be colorless and ellipsoid or spherical in shape measured 9.5 х 8.5μm. The oocyst wall consists of two layers, micropyle and micropyle cap were not observed in these oocysts, and wall forming bodies were arranged at the periphery of oocyst directly under the developed oocyst wall. Sporulated oocyst contains 8 sporozoites
filling the entire cavity of the oocyst without sporocyst formation. Fully sporulated oocysts were excreted in the faces of infected snails from 14-18 day P.I., these oocysts measured 9.5х8.5μm. Micropyle was absent and while a residual body was observed.