A 43-year-old, bisexual, black man with acquired immunodeficiency syndrome (AIDS), detected by CD4 lymphocyte criteria alone, presented with low-grade fever, chills, malaise, and watery diarrhea of 2 days' duration. Over the next 5 days, he developed a fulminant septicemia-like illness with progressive hypotension, disseminated intravascular coagulation, and very high serum lactic acid dehydrogenase (2,150 U/L) and serum creatine phosphokinase (5,395 U/L) levels, and died. The cause of this illness was not clinically apparent. A bone marrow biopsy performed on the day of his death revealed intracytoplasmic clusters of 3 microns long, oval, basophilic organisms, the exact nature of which was not evident by light microscopy. The diagnosis of disseminated toxoplasmosis (DT) was made only after electron microscopic study of the bone marrow revealed organisms with features typical of Toxoplasma gondii tachyzoites. These features included a multilayered pellicle, a pointed anterior end containing a conoid, up to nine rhoptries, sparse micronemes, and a posterior end containing a nucleus. Some of the organisms had divided by internal budding or endodyogeny. This case illustrates the value of transmission electron microscopy in making the diagnosis of DT.