The distribution and apparent habitat preference of the common mummichog (Fundulus heteroclitus) in south-western Spain were examined during summer–autumn 1996. This introduced species was more or less continuously distributed along the Atlantic coast of Spain, being more abundant in sites near the coastline (usually <10km inland), mainly in four extensive marshes. The species preferred marsh-related mesohabitats, such as salt lagoons, saltmarsh fish ponds and marsh channels, both natural and man-modified.Fundulus heteroclituswas mostly found at salinities >25. It was the most frequently captured fish species, occurring at 81 of the 272 sites sampled; their frequency of occurrence was almost twice that of the second ranked species (Gambusia holbrooki). However, in over 80% of cases,F. heteroclituswas found alone or with only one sympatric fish species, which usually belonged to a group composed ofGobius niger,mugilids,Anguilla anguilla, Blenniussp.,Lebias ibera, Pomatoschistussp. andDicentrarchus labrax. Finally, the origin and dispersal of mummichog in the Iberian peninsula and the potential effects of this species on native fish populations is discussed.