Through a series of odor preference and sexual activity tests, the response of male western mosquitofish, Gambusia affinis, to female odors was investigated. Male mosquitofish prefer female odors to male odors and plain water, indicating that female odors function as a sex attractant. In odor preference tests, males did not show preferences among odors from nongravid, gravid, and parturient females. However, odors from females in different stages of the reproductive cycle induced different sexual activities from males. Odors from gravid females increased copulation trials, whereas those from parturient females increased male-male interactions without changing copulatory activity. These results suggest that (1) female western mosquitofish may produce sex pheromones and (2) female pheromones from different stages of the reproductive cycle stimulate different male sexual activities, suggesting that males may discriminate parturient females from others based on pheromonal output. In populations where the operational sex ratio (OSR) is female-biased, the discrimination of reproductive stages of females by males could increase male reproductive success by allowing males to avoid spending mating efforts with females whose eggs they are unlikely to fertilize.