Fundulus is a genus of ray-finned fishes in the superfamily Funduloidea, family Fundulidae (of which it is the type genus). It belongs to the order of toothcarps (Cyprinodontiformes), and therein the large suborder Cyprinodontoidei. Most of its closest living relatives are egg-laying, with the notable exception of the splitfin livebearers (Goodeidae).

They are usually smallish; most species reaching a length of at most 4 in (10 cm) when fully grown. However, a few larger species exist, with the giant killifish (F. grandissimus) and the northern studfish (F. catenatus) growing to twice the genus' average size.

Many of the 40-odd species are commonly known by the highly ambiguous name "killifish" (the general term for egg-laying toothcarps), or the somewhat less ambiguous "topminnow" (a catch-all term for Fundulidae). "Studfish" is a quite unequivocal vernacular name applied to some other Fundulus species; it is not usually used to refer to the genus as a whole.

Fundulus have evolved to occupy a wide range of aquatic ecosystems, including marine, estuarine, and freshwater, making it a good comparative model system for studying evolutionary divergence between marine and freshwater environments. To assist with this research, Oxford Nanopore long-read reference genomes have been sequenced for F. xenicus, F. catenatus, F. nottii, and F. olivaceus.

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