The patterns of spreading and seasonal variation of population density of Biomphalaria alexandrina, the snail vector of Schistosoma mansoni in Egypt, in correlation with some environmental parameters were studied for 1988-1990 in four irrigation canals in Giza and Qalyoubiya Governorates. Results shown that the spreading patterns of snails along canals are changeable because of water current and irrigation activities. The snail population density showed two peaks, different in height, in April-May and November-December separated by two bottoms following the Winter Closure and during the hot summer season. The height of the peaks appears to be dependent on the extent of the "Winter Closure" and the prevailing water temperature. Two snail sampling tools, the dip-net and drag scoop, were used concurrently in this study. No considerable difference in sampling efficacy was recognized between these tools in the case of Biomphalaria. However, the dip-net appears to be more efficient in sampling other pulmonates while the drag scoop is more efficient in sampling prosobranchs. It is also realized that double sampling of 50% of sampling sites the next day should be satisfactory as a quality control system in snail population studies.