The mating season in the northern watersnake (Nerodia sipedon) occurs in the spring during vitellogenesis. Courtship and mating occur primarily in trees and bushes which overhang the water, but may also occur on the stream bank. Watersnakes access these basking/mating sites from the water. Thus, the terrestrial trailing of a female, typical of most of the snakes studied, cannot occur. Based on field observations, only about 10% of female N. sipedon are in estrus (attractive to males via pheromones) on days with appropriate temperatures from late April to mid-June. Females in estrus vary in attractiveness and receptivity to males. In field experiments, males are able to locate caged females apparently via volatile pheromones. We suggest that watersnake pheromones are heavy, volatile compounds that accumulate in the air above the surface of the water and that males climb branches and systematically search for females in the vicinity of pheromone concentrations. In addition to pheromones, males use other means to locate females including systematic searching of bushes and movement towards moving snakes. Courtship may last from several minutes to more than 2 h. Most courtships that occur late in the mating season do not result in copulation.