The dominant grasses and sedges on the tidewater marshes in Worcester County, Maryland, that were ditched for mosquito control are Spartina alterniflora, S. patens and Scirpus olneyi. S. alternifolia usually occurs in solid stands nearest the main body of salt water, S. patens on higher levels away from the water, and Scirpus olneyi near the border of higher levels where the fresh water enters the marsh. Collections developed the fact that Aedes sollicitans, A. cantator and Culex salinarius are important breeders in S. olneyi. The main marsh mosquitoes, A. sollicitans and A. taeniorhynchus, occur mainly in Spartina marshes. Water levels were lowered by ditching to a greater extent in S. olneyi than in any of the other 2 types of marsh covering. The following is the order in which various spp. of mosquitoes occur in relation to a decreasing scale of salt content in the water, the range being from 37 to 4.75: A. sollicitans, A. taeniorhynchus, Anopheles crucians, Aedes cantator, C. salinarius, Aedes canadensis, and C. apicalis. The relation of ditching to duck food and duck breeding is also considered.