Cyprinodon dearborni, Poecilia sphenops and Rivulus marmoratus seem to fill almost the same niche. In most of the landlocked bays, lagoons or pools the three species were not found together. In only two landlocked locations two of the species were found together. In the locations with an open connection with the sea all three species, or at least two were present. In landlocked waters interspecific competition seems to play an important role as the niches of the three species overlap to a large extent. Environmental factors of importance appeared to be: 1) Plant or algal growth is required by Poecilia and Rivulus. 2) Low salinities seem to be of advantage to Poecilia and Rivulus, supersaline environments seem to favour Cyprinodon. 3) Oxygen depletion may threaten Poecilia more than the others 4) Cyprinodon is able to take any food of suitable size but its main food source is formed by unicellular cyanophyceans, a source that is almost inexhaustible. Poecilia, although omnivorous, is more particular in its food choice; Rivulus is carnivorous and its population depends on the availability of live food. 5) Cyprinodon does not prey upon the other two species. Poecilia preys upon the eggs and fry of the others, and Rivulus also takes juveniles. 6) Cyprinodon is hardly ever cannibalistic; it may only consume some of its eggs. Cannibalism in Poecilia and Rivulus is of great importance. In adverse conditions, it keeps the populations small, which seems to be an advantage in small lagoons or pools. 7) Ovoviviparity and hermaphroditism in Poecilia and Rivulus may have a special advantage in maintaining very small populations. 8) In isolated waters, predation by other fish species or by birds seem to be of little importance. 9) Mass mortality because of parasitism is an exception. 10) All three species are very able to flee from adverse conditions. If a population is destroyed repopulation will take place from the sea where small, but stable populations are present. 11) Many of the characteristics mentioned are of special advantage in island environments.