Six collections of Biomphalaria pfeifferi were made in the Nairobi and Machakos areas of Kenya. Individual cercarial shedding (Method A) showed that 6.9% (65/938) of the snails had mature 'patent' Schistosoma mansoni infections (range 3.1--34% in the six collections). Crushing (Method B) about one-third of the remaining snails detected no additional mature infections but yielded 1.4% (four snails from one collection) with secondary sporocysts from immature 'prepatent' infections. Weekly shedding observations for five weeks (Method C) on the remainder of the snails showed that 6.9% (40 snails from four collections--range 1.5--30%) shed cercariae maturing from infections immature at the time of collection. As Method B detected no cercariae, Method A apparently detected all 'patent' infections reliably, despite the stress suffered by the snails before examination. Method C was simpler and more efficient than Method B in the detection of infections which were immature at the time of collection. Judging from the rate at which infections matured in Method C, field snails in heavily endemic areas are subjected to pulses of infection rather than to a continuous flow of miracidia. The 'patent' infection rate was not correlated to the 'prepatent' infection rate. Some of the implications of these findings to mathematical models of schistosome transmission are discussed.