The repellent deet (N,N-diethyl-3-methylbenzamide) was tested against the mosquito Aedes dorsalis in a coastal salt marsh in California. The experimental design incorporated a multiple regression model, sequential treatments and a proportional end point (95%) for protection time. The ED95 (95% effective dose) and 4-h ED95 were estimated at 0.05 mg/cm2 and 0.09 mg/cm2, respectively. The 0.05 mg/cm2 protection time and 0.10 mg/cm2 protection time were estimated at 0.2 h and 4.4 h. The decay constant and half-life were estimated at 0.17 h-1 and 4.1 h. The design and analysis of repellent field trials are discussed.