1. Reproduction in the snail Biomphalaria glabrata, a vector of Schistosoma mansoni, was measured by egg production and other parameters such as number of clutches, eggs per clutch and egg hatching, under laboratory conditions. 2. Reproductive rates were highest in terms of total number of eggs, clutches and hatching under constant illumination, followed by conditions similar to the natural rhythm of illumination. The greatest reduction in reproduction occurred in constant darkness. In summary, reproductive rate was dependent on both the intensity of absolute illumination and the schedule of illumination exposure. 3. Seasonal variation in reproductive rate appeared to vary inversely with local temperature in Belo Horizonte, situated in a tropical zone (latitude 20 S) owing probably to a temperature preference of 20 to 25 degrees C by the snail. The highest reproductive rates occurred in winter, except for the parameter eggs per clutch which was maximal in autumn. Snail reproduction appeared to be most reduced during summer. There was no linear correlation between reproductive rate and temperature.