(species Gambusia affinis), live-bearing topminnow of the family Poeciliidae (see live-bearer), native to freshwaters of the southeastern United States but widely introduced in other parts of the world for mosquito control. The hardy mosquito fish, which has a prodigious appetite for mosquito larvae, is usually light grayish but may be spotted or blotched with black. The female reaches a length of about 6.4 cm (2.5 inches), the male about 4 cm (1.5 inches).