Bacillus sphaericus was used to control Culex pipiens breeding in a water treatment settling basin near Montpellier, France. Four treatments with 4 liters/ha (3.6 lb/acre) of commercially available B. sphaericus formulation (Vectolex) reduced the larval population of Cx. pipiens with 50-600 spores of B. sphaericus/ml recorded in the treated portions of the water plant 14 days following the last treatment. Natural recycling of the bacteria was shown to take place at the water surface where germination of spores was evidenced. The appearance of vegetative forms of B. sphaericus may be linked to the passage through larval guts of several filter-feeding arthropods.