Isospora belli has recently been recognized as an opportunistic protozoan pathogen in patients with the acquired immunodeficiency syndrome (AIDS). Although I. belli rarely causes diarrhea in patients with AIDS in the United States, we have documented isosporiasis in 15 percent (20 of 131) of such patients in Haiti. The infection was associated with chronic watery diarrhea and weight loss that was clinically indistinguishable from disease caused by the related coccidia cryptosporidium. No demographic or laboratory data distinguished the patients with AIDS and isosporiasis from those with either cryptosporidiosis or other opportunistic infections. Neither I. belli nor cryptosporidium was detected in stool samples from 170 healthy siblings, friends, and spouses of the patients with AIDS. In all patients with isosporiasis, diarrhea stopped within two days of the beginning of treatment with oral trimethoprim-sulfamethoxazole. Recurrent symptomatic isosporiasis developed in 47 percent of the patients, but it also responded promptly to therapy with trimethoprim-sulfamethoxazole. We conclude that isosporiasis is common in Haitian patients with AIDS, and that it responds to therapy with trimethoprim-sulfamethoxazole but is associated with a high rate of recurrence.