The role of Culex pipiens larval ingestion rate in the effectiveness of three Bacillus sphaericus strains, namely 2362, B1-634BNI and 6491 was assessed. Two concentrations, namely LC₅₀ and LC₈₀, were tested. Studied parameters included bacterial exposure time and mosquito larval density. Exposure time periods were 0.5, 1, 2, 4, and 24 h. Larval density varied by exposing different numbers of larvae yielding larval density of 0.1, 0.2, 0.4, 0.8 and 1.6 larva/ml. Larval density also varied by exposing 20 larvae in different water volumes yielding larval density of 0.8, 0.4, 0.2 and 0.1 larva/ml. Strain B1-634BNI was the most active (LC₅₀ = 0.02 ppm) among the tested strains. Mortality rates of larvae varied at different exposure periods and with the different larval densities. Mortality rates of Cx. pipiens larvae increased with the increase of the exposure period of the larvae to the bacterial suspension of strain B1-634BNI either at its LC₅₀ or LC₈₀. As larval density increased, mortality rates of larvae decreased when treated with each of the three tested bacterial strains whether at its LC₅₀ or LC₈₀. Adjusting bacterial application in control programmes according to those elements is thus indispensable to ensure a satisfactory result.