ABSTRACT.The larval susceptibility toBacillus sphaericusstrain 2362 of the non‐man‐biting mosquitoCulex cinereusand the urban filariasis vectorCulex quinquefasciatus, two competitor mosquitoes in polluted habitats, was compared. In the laboratory, both species ingested a similar amount ofB.sphaericusspores when fedc.2×l05spores per ml for 30 min. However, in the same experiment, third‐instar larvae ofCx quinquefasciatuswere reduced by 98% at 24 h exposure whileCx cinereuslarvae were only reduced by 6% at 72 h. In the field, preimaginal populations ofCx cinereusingested, within a week, more than 99% of the applied spores, but showed no significant decrease through 14 days in cesspools treated at 10 g/m2of a flowable concentrate ofB.sphaericus2362, containing 2times 1010spores/g.It is proposed that specific biological control ofCx quinquefasciatuscould result from appropriate treatment of breeding‐sites with larvicidalB. sphaericusand competitive displacement byCx cinereusor other mosquitoes with larvae that are more tolerant ofB.sp