Introduction Interest in the relationship of the lipophilic yeastlike Pityrosporum orbiculare to the disease tinea versicolor has increased1-3 since the isolation and detailed description of the organism by Gordon4,5 in 1951. Except under unusual conditions,3 inoculation with P. orbiculare has failed to produce microscopic or clinical evidence of tinea versicolor. However, the similarity of the budding cells of P. orbiculare to those of Malassezia furfur and the ease with which P. orbiculare can be cultured from patients with tinea versicolor have prompted further consideration of their etiologic relationship. This paper presents immunofluorescence studies, which, it is hoped, will shed some light on the problem. Materials and Methods Antiserum.— High titer agglutinating antibodies to P. orbiculare were found in the sera of 2 patients with extensive tinea versicolor. One of these 2 sera was free of minor antibodies to Candida albicans, Candida parapsilosis, Candida krusei, Pityrosporum ovale, and