A congenital murine model of toxoplasmic retinochoroiditis was employed to study the ultrastructural pathology of retinal parasitization by Toxoplasma gondii. Forty-two eyes from infected mice (18-22-weeks-old) and 24 eyes from control animals were studied by light microscopy (semithin sections). Twenty-six of the eyes from infected animals and six from the control group were subsequently selected for transmission electron microscopy. Control tissues showed no significant abnormality. The pathological changes in diseased tissues ranged in severity from low-grade mononuclear cell infiltration in the subretinal space to complete destruction of the outer retina, the retinal pigment epithelium and the choroid in the presence of a granulomatous inflammatory reaction. Phagocytosis of photoreceptor outer segments by macrophages was observed. Both macrophages and lymphocytes appeared to mediate photoreceptor lysis in eyes which were moderately affected by the disease. Severely affected eyes exhibited vasculitis and inflammatory cell invasion into the vitreous. A lymphoplasmacytoid cell infiltrate was present in the outer retina and choroid in these eyes. There was no evidence that Toxoplasma cysts provided foci for inflammatory cell attack.