Mature male mosquitofish possess a highly modified anal fin called a gonopodium that develops from an unmodified female-like anal fin under the influence of endogenous androgens at sexual maturity. Although females do not normally develop gonopodia, their anal fins can be induced to develop into gonopodium-like structures by the administration of androgens. We used computer image analysis techniques to quantify the morphological changes in the anal fins of normal male and female mosquitofish during growth and development and the changes that occur in females exposed to an androgen, 11-ketotestosterone. The mosquitofish anal fin is a useful bioassay system for quantifying the effects of known or suspected environmental androgens. © 2001 Academic Press.