From March 1991 to February 1992, 276 stool samples from 131 patients with the acquired immunodeficiency syndrome (AIDS) (average, 2.1 samples per patient) and stool samples from 81 presumed immunocompetent individuals were studied for the prevalence of Cryptosporidium sp. and Isospora belli. The techniques employed were a modified formol-ether concentration, carbol (phenol) auramine staining, and a modified Kinyoun acid-fast method. The prevalence of both coccidia among AIDS patients was 25.9% (34 of 131). Cryptosporidium sp. oocysts were demonstrated in 25 (19.1%) AIDS patients and I. belli in 13 (9.9%). Coinfection for coccidia of both species was found in 4 (3%) AIDS subjects. Among the presumed immunocompetent individuals, coccidia were not demonstrated in their 81 stool specimens. This study, like others, confirms the worldwide importance of these coccidia among AIDS patients and the necessity of suitable techniques for demonstration of oocysts in stool samples.