Coccidia of the lacazei group of the genus Isospora were cultured in vitro. Isospora from the English sparrow, Passer domesticus domesticus, developed in primary cultures of embryonic chick and canary cells grown in Leighton tubes in medium 199, Hanks' base with 10% calf serum. When sporozoites were added to tubes containing chick or canary cells, trophozoites were first observed at 3 hr, immature schizonts with refractile globules and mature schizonts at 24 hr, and immature schizonts without refractile globules at 48 hr. Evidence that at least 2 asexual generations developed is presented. Mature schizonts accounted for 66% of the total developing parasites in chick cells on day 5 and for approximately 90% of the total parasites in canary cells on days 4, 5, and 6. Many more parasites developed in canary cells than in chick cells. The first cultivation of a coccidian in cell culture was reported by Patton (1965), who grew asexual stages of a chicken coccidian, Eimeria tenella, in monolayers of a variety of cells. He observed the development of at least one asexual generation. Doran and Vetterling (1967a, b, 1968) cultured other Eimeria species from turkeys and chickens in a variety of cells through at least one cycle of asexual development. Hammond and Fayer (1968) reported growing second-generation schizonts of Eimeria bovis, a parasite of cattle, in an embryonic bovine tracheal cell line and summarized their work and the work of others in cultivation of Eimeria in vitro. Development of sexual stages in cell culture was first reported by Strout and Ouellette (1969), who obtained development of mature macroand microgametes in primary chick embryonic kidney cells inoculated with sporozoites of E. tenella. More recently Doran (1970) reported development of the same parasite from sporozoites to viable oocysts in cell cultures prepared so that cell aggregates were abundant. Although species of the coccidian genus Eimeria have previously been grown in cell cultures by several investigators there is only one report of cultivation of an Isospora in vitro (Sheffield and Melton, 1970). These workers cultured asexual stages of Toxoplasma gondii from sporozoites obtained from an Isosporalike oocyst from cats. Other workers have demonstrated that T. gondii can be transmitted by an oocyst resembling Isospora bigemina found Received for publication 9 December 1969. in cats (Frenkel et al., 1970; Hutchison et al., 1970). The present paper reports the first cultivation in vitro of an Isospora of the lacazei group from English sparrows (Passer domesticus domesticus). MATERIALS AND METHODS Cells for culture were obtained by trypsinization of 10-day-old chick embryos and 9to 13-day-old canary embryos. Both types of cells were suspended in growth medium 199 in Hanks' balanced salt solution with 10% calf serum, penicillin (100 units/ml), streptomycin (100 ,/g/ml), and phenol red (0.02 g/liter). Leighton tubes containing 10.5by 35-mm cover slips were each inoculated with 0.8 ml of a cell suspension containing 1.6 X 10' cells. Isospora oocysts were obtained from feces of English sparrows trapped on the Galveston campus of the University of Texas Medical Branch. Sparrows were maintained in air-conditioned quarters and fed chick starter mash. Clean newspapers were placed on the floor of the cages between 2 PM and 3 PM. Water was poured on the papers to prevent drying of feces passed in the night. Feces were collected as soon as possible the next morning since most of the oocysts are passed between 3 PM and 11 PM (Boughton, 1933). Approximately 2 parts of 2.5% (w/v) potassium dichromate were mixed with 1 part of feces in a Waring blender for 1 min, strained through 2 layers of gauze, and poured into petri dishes in layers not over 5 mm deep. After 6 days at room temperature, fecal mixtures were pooled, diluted with an equal volume of water, and stored in the refrigerator until needed. Measurements were made on oocysts collected and sporulated as above using an ocular micrometer at a magnification of 400 X. Oocysts were recovered from fecal mixtures by a flotation method similar to that described by Doran and Farr (1962) except that the flotation solution contained 180 g cane sugar mixed with water to make 1 liter (sp gr 1.08). Oocysts were