Although male mosquitofish invest little in their progeny, the simultaneous availability of many potential mates and the large variance in their quality are expected to favour the evolution of mechanisms of male mate choice. Males of Gambusia holbrooki were shown to exhibit sexual preferences when presented simultaneously with two potential mates. When females differed in size, males showed more sexual activity directed at the larger of the two and the time spent with this female increased with the difference between the two females. This preference, however, was lower than expected if males spent time in direct proportion to the difference in fertility between females. In the second part of the study, females which had just delivered were found to be clearly preferred over gravid females. Attractiveness of females dropped after the third day, and disappeared in the sixth day after parturition. The results are discussed in relation to the probable occurrence of sperm competition and sperm displacement in this species.