Guppy Poecilia reticulata (Cyprinodontiformes: Poeciliidae) and the mosquitofish Gambusia affinis (Cyprinodontiformes: Poeciliidae) are small ovoviviparous fish which have established their introduced populations on Okinawa Island. They are likely in a competitive relationship, because their distributions are mutually exclusive on Okinawa. The underlying mechanism is, however, unknown.  Our field surveys suggested that the mosquitofish initially became established on Okinawa Island, before being replaced by the more recently introduced guppies. Moreover, our laboratory experiments suggested that reproductive interference was a mechanism underlying this species exclusion. The presence of guppy males led to a decrease in fecundity of the mosquitofish females, whereas fecundity of guppies was not affected by the presence of the mosquitofish males. We discuss the possibility to utilize this reproductive interference to eradicate the pest mosquitofish.