Previous studies have shown the mosquitofish (Gambusia affinis) to be euryhaline, but it is not known whether inland freshwater populations show this attribute, and no studies have examined whether sexes or sizes respond differently to increasingly salty environments. Allowing fishes to gradually acclimate to increasing salinity, I found no effects of sex or size, that most mosquitofish from an inland population can tolerate salinities equal to seawater, and that most individuals can survive much saltier conditions. Tolerance to salinity may be a residual attribute of ancestors that dispersed through marine environments from Central America.