Development of male zebrafish (Danio rerio) courtship behavior was studied following estrogenic disruption of sexual differentiation. Sixty zebrafish were exposed at 28 degrees C to 5 ng/L (nominal concentration) of 17alpha-ethinylestradiol (EE2) from the egg stage until adulthood at four months of age, resulting in a female-biased sex ratio. Twenty-five EE2-exposed phenotypic female zebrafish were subsequently held in clean water for eight months. During this period, eight phenotypic males developed. These phenotypic males demonstrated significant behavioral aberrations and a low fertilization rate compared to control males.