The binary toxin of Bacillus sphaericus strains forms a crystal in sporulating cells, while the mosquitocidal toxin is located in the cytoplasm of vegetative cells. The distribution of binary toxin (btx) and mosquitocidal toxin (mtx) genes in 53 strains of B. sphaericus was determined by hybridization of specific gene probes to chromosomal DNA in Southern blots. btx genes were found in all strains of serotype 5a5b examined and in some strains of serotypes 1a, 3, 6, 25, and 48, while mtx genes were detected in strains of serotypes 1a, 2a2b, 5a5b, 6, 9a9c, 25, and 48. Serotype 26a26b strains lacked both toxin genes, as did some strains of serotypes 2a2b, 3, 6, and 48. Partial DNA sequences of btx genes from five strains, together with published sequences, revealed four types of toxin among mosquitocidal B. sphaericus strains. most of the 42-kDa toxin gene of btx was identical in strains from serotypes 1a, 3, 6, and 48, and the gene is here classified as a type 1 btx gene. A serotype 3 strain isolated in Singapore possessed a unique 42-kDa toxin gene, here designated type 4; while the btx genes from strains of serotypes 5a5b and 25 are referred to as types 2 and 3, respectively.