Development from inoculated sporozoites to unsporulated oocysts of Isospora suis Biester, 1934 is described in a swine testicular (ST) cell line. Sporozoites penetrated ST cells within 1 hr postinoculation (PI). Development was initially by endodyogeny to produce binucleate type I meronts and type I merozoites. Division by endodyogeny continued during the 13-day observation period and type I merozoites were the developmental stages most abundant at observation periods >3 days PI. Mutinucleate type II meronts and type II merozoites were first observed 7 days PI. Gamonts and oocysts were present 12 days PI. Oocysts did not sporulate in vitro. The ultrastructural features of stages were similar to those that occur in the pig host.