Coccidian oocysts resembling those of Isospora bigemina were excreted by cats fed Toxoplasma. In order to identify these oocysts with Toxoplasma infectivity a number of critical comparisons were made. The appearance of oocysts and Toxoplasma infectivity was simultaneous in the feces of 23 of 24 adult cats, 3–5 days after feeding of Toxoplasma cysts; in the feces of 4 out of 9 cats, 7–10 days after feeding of trophozoites; and in 8 out of 17 cats, 20–24 days after feeding of cat feces containing oocysts. Oocysts and infectivity were present in similar numbers, and they disappeared simultaneously from the feces of cats. Oocysts and infectivity were also observed simultaneously in the feces of 9 kittens, 1–2 days old, fed Toxoplasma cysts. Oocysts could not be separated from infectivity by filtration, by continuous particle electrophoresis, or by density gradient centrifugation. Excystation of oocysts was followed by an increase in titer of Toxoplasma infectivity.

Unsporulated oocysts in fresh cat feces were noninfectious to mice, but oocyst sporulation was associated quantitatively with the development of infectivity at different temperatures and conditions of oxygenation. Maximum oocyst sporulation at 48 hr correlated with the development of maximum Toxoplasma infectivity.

1 and 2% sulfuric acid, and 2.5% potassium dichromate were found to be the best preservatives for sporulation of oocysts and for the development of Toxoplasma infectivity. Low sporulation rates in 0.1% formalin, 20% ethanol, and in water were associated with low infectivity in these reagents. Neither Toxoplasma infectivity nor oocysts developed in 0.3% formalin, 1% ammonium hydroxide, or 1% iodine in 20% ethanol.

Oocysts, sporocysts, and sporozoites were stained specifically with Toxoplasma antibody in the indirect fluorescent antibody test. Typical coccidian stages, schizonts, and male and female gametocytes were found in the epithelium of the small intestine of kittens fed Toxoplasma cysts. The classification of T. gondii is discussed in relation to that of other isosporan coccidia of cats and dogs. The term "Toxoplasma oocyst" is introduced and Toxoplasma is classified in the family Toxoplasmidae of the suborder Eimeriina. The species Isospora bigemina is restricted to dogs, and I. cati to cats. I. felis and so-called I. rivolta from cats were noninfectious to dogs, and did not confer immunity to subsequent infection with I. canis and I. rivolta from dogs.