The principal purpose of this study was to ascertain what effect on wildlife, if any, would result from the use of the new insecticide, Strobane, for mosquito control on tideland areas. Comparisons were made with DDT and BHC (43 per cent gamma isomer) commonly used in control operations. The investigation was carried out on the tidal marshes of Bombay Hook National Wildlife Refuge near Smyrna, Delaware. Four areas, all similar in habitat, were chosen-three as test plots for Strobane, BHC, and DDT, respectively, and the fourth as an untreated check. The insecticides in oil solution were applied by airplane at the rates of 0.1, 0.2, and 0.3 pound per acre for gamma isomer of BHC, DDT, and Strobane, respectively. The first application was made on the morning of July 27; and the second, on the evening of August 23, 1955. To assay the results of spraying, 14 testing devices were set up in each area. They consisted of cages, traps, and microscope slides placed in the streams and ponds. The estuarine fishes, Fundulus heteroclitus, Cyprinodon variegatus, Leiostornus xanthurus, and Ailugil curemu; blue crabs, Callinectes sapidus; fiddler crabs, Uca pugnux, Uca minux, and Sesarma reticulaturn; and certain sessile organisms were included in the tests. Analyses of variance on fish and blue crabs showed no significant difference between deaths occurring in treated and control plots, nor among the three treatments. Differential mortalities were suffered by fish caged in streams and ponds. Greater numbers died in the BHC-treated streams and in the DDT-treated ponds. Local concentrations of insecticide appeared to be the cause, although the magnitude of kill was not significantly greater than in control areas. Field observations and crab-pot counts showed that both the fish and blue crabs avoided the sites of high insecticide concentration. Certainly the majority of the free-living individuals in the treated areas were able to survive the sprays, and at the level tested showed no acute toxicity from single applications. Of all the marsh inhabitants tested, the marsh fiddler crab, U. pugnux, was most obviously affected by the three toxicants. Mortalities of 68 to 80 per cent were sustained by this species in cage tests, as against 16 per cent for controls. Greater losses were observed in the BHC area. Under the same conditions, U. minax and S. reticulutum were relatively unaffected. The economic and ecological importance of the marsh fiddler should not be overlooked. It constitutes one of the dominant crustaceans in the area and is fed upon by birds and mammals. Sessile organisms were collected on slides, but no accurate interpretation could be made. Gross observations indicated no apparent harm to birds and mammals. Due to circumstances beyond control, a third application, and a chance to measure chronic toxicity, was not achieved. No study on reproductive stages or immature animals was undertaken. The total interaction of changes produced by the insecticide sprays was not evaluated. However, it can be stated that, under the conditions of the test, no consequential damage from the two applications was observed, with the exception of that to the marsh fiddler crabs.