For 3 years (1972-75) the numbers of Gambusia puncticulata and other minnows in a Grand Cayman mangrove swamp were monitored by daily trapping. Concurrently the swamp's water levels were measured and the occurrence of immature Aedes taeniorhynchus recorded. The minnows were unable to control this mosquito due to 3 effects of flooding: 1) immediate hatching of mosquito eggs, 2) dilution of the minnow population and 3) delayed increase in G. puncticulata numbers and lack of increase among other minnow species. Additionally, fish numbers were greatly reduced whenever the swamp drained.