Effects of weather, season, sex, and age upon the trap responses of wild cottontails (Sylvilagus floridanus) were evaluated using data obtained from September through March in two trapping seasons. Except for adult males, rabbits became more trappable during weather conditions prevailing when barometric pressure was high. Adult males became more trappable during weather including precipitation. Overall, 13 percent of the variation in trap success was associated with weather factors. Although female rabbits were more trappable than males, the difference was not statistically significant. Rabbits were most trappable at the age of 4-5 months and thereafter became less trappable, especially as adults. A uniform probability of capture did not exist among individuals within sex-age classes. The animals were most trappable during October through December, less trappable during September and January, still less trappable during February, and least trappable during March. Statistical models including sex, age, and month accounted for about 20 percent of the variation among trap responses. Trap success cannot be a sensitive index to population density and sex-age structures of trapped samples will usually be biased toward overestimation of the proportions of juveniles and females in rabbit populations. Studies of wild populations of cottontails often include periodic trapping with box traps. Analyzing the resulting data usually involves the questionable assumptions that trapped animals are randomly drawn from the population and that success in trapping can be used as an index to population density. Geis (1955) demonstrated that individuals within rabbit populations were not 1A contribution from Federal Aid Project W-66-R, the Illinois Department of Conservation, the U. S. Bureau of Sport Fisheries and Wildlife, and the Illinois Natural History Survey, cooperating. 2 Present address: School of Forestry, University of Montana, Missoula, 59801. equally trappable, and the aggregate of several other studies, mentioned below, has suggested that variation among trap responses of rabbits is associated with weather, season, the sexes and ages of the animals, habitat, the density of the rabbit population, and the number and distribution of traps. Effects of the first three fact rs upon trap responses of cottontails are