Intersex in gonochoristic species is a reproductive abnormality in which an individual produces gametes of the type normally associated with the opposite sex. To investigate the prevalence of intersex in male lionfish Pterois spp., 884 individuals were collected from the Bahamas and North Carolina between 2004 and 2008. Histological sections of testes were scored for the presence or absence of oocytes. In intersex individuals, the number of oocytes, size, and developmental stage was recorded. The prevalence of intersex was approximately 15% and, along with all other measured metrics describing the condition, did not significantly differ between the 2 locations. Most of the intersex males displayed only primary-stage oocytes embedded in structurally normal gonadal tissue. However, 3 fish from the Bahamas exhibited oocytes of all developmental stages and complete restructuring of the gonadal tissue that is atypical of both male and female lionfish. This study is the first extensive report of intersex in lionfish, and the first in a gonochoristic reef fish.