Seasonal changes in populations of Bulinus senegalensis were studied in temporary pools on the Senegambian plateau. The first snails to appear had survived aestivation and had a mean height of 3 mm. Thereafter two patterns of change in snail numbers and size were observed probably related to differences between pools in snail mortality rates. The prevalence of patent schistosome infections appeared to be related to the inferred differences in snail mortality rates. The prevalence and intensity of human Schistosoma haematobium infection was greatest in villages near large pools where snail populations had relatively low schistosome infection rates compared with that in a small, shallow pool. Differences in patterns of water use are an important determinant of the intensity of human infection as well as differences in the infection rate of snails in the nearby pool. Differences between years in the rate of acquisition of S. haematobium by children were observed, with little transmission occurring in a year when mid-season rainfall was deficient. There were significant differences between villages in the annual variations of transmission.