Five areas of irregularly flooded salt marsh were studied to determine effects of ditching on mosquito populations. Principal species were Anopheles atropos, Anopheles bradleyi, Aedes sollicitans, and Aedes taeniorhynchus. Present in lesser numbers were Culex salinarius, and Psorophora confinnis. Data on both plant cover and on the abundance of mosquito larvae showed such wide variation in both ditched and unditched sections that no significant differences between sections could be detected. Percentage of Aedes and Psorophora larvae taken in samples tended to be directly proportional to mean slope of the breeding depression and inversely proportional to its frequency of flooding. Conversely, the relative abundance of Anopheles and Culex species was directly proportional to the frequency of flooding. Relative abundance of species in the ditched sites was more affected by the mean slope, frequency of flooding and the ratio of frequency of flooding/mean slope than in the unditched sites. (Modified author abstract)