Japanese isolates of Bacillus thuringiensis were screened for larvicidal activity against the mosquito Anopheles stephensi, the urban malaria vector of the Indian subcontinent. Among more than 30 strains identified, larvicidal activity causing >80% mortality in 72 h was demonstrated for 41/1449 (2.8%) isolates. The majority of strains and isolates (97.2%) exhibited little or no larvicidal activity. Anopheles-active strains belonged to more than 12 H serotypes, especially H3ade (serovar fukuokaensis) and H44 (serovar higo). SDS-PAGE profiles of inclusion proteins showed 4 distinct types among 6 active strains examined. The most active Japanese isolates were H20 strain 89-T-34-14 (LC50 4.4 μg/ml) and H44 serovar higo strain 74-E-45-24 (LC50 7.6 μg/ml), respectively, 13-fold and 23-fold less active than the international standard H14 serovar israelensis (LC50 0.33 μg/ml).