It has been demonstrated that Toxoplasma is a cause of "chorioretinitis" in adults. The frequency of toxoplasmic ocular disease and the manner of diagnosis of this entity, two questions which are interdependent, have been clarified in part. It is the aim of this paper to summarize the evidence incriminating Toxoplasma and to discuss the modes of pathogenesis. On this basis, we can discuss the usefulness of various diagnostic tests, their method of application, and the rational approaches to the chemotherapy of the disease which are available at present for individual patients. Evidence Linking Toxoplasma with Retinochoroiditis in Adults In a clinical syndrome which presumably may be brought about by a variety of agents, the determination of the etiology in individual patients presents a most difficult diagnostic problem. This is particularly so in "chorioretinitis," since it is generally not feasible to isolate agents from the eyes of living patients. Various agents