Bacillus thuringiensis var. israelensis (BTI), serotype 14, which produces parasporal crystals toxic to certain dipteran larvae, was analyzed by agarose gel electrophoresis and found to contain a complex plasmid array. Eight plasmids were detected, with approximate sizes of 3.3, 4.2, 4.9, 10.6, 68, 75, 105, and 135 MDa, as well as a plasmidlike linear DNA element of approximately 10 MDa. Partially cured mutants of BTI implicated the 75-MDa plasmid in crystal production. Fifteen independently isolated acrystalliferous (Cry-) mutants were found to lack this plasmid. In plasmid transfer experiments, several of the BTI plasmids transferred into a plasmid-free, Cry- BTI recipient, but only transfer of the 75-MDa plasmid converted the recipient to crystal toxin production. The presence or absence of mosquito-toxic activity in all Cry+ and Cry- variants of BTI was confirmed by bioassay of sporulated cultures against larvae of Aedes aegypti. Southern blot analyses revealed that in one unusual Cry+ variant in which no 75-MDa plasmid band was detectable, plasmid sequences were still present, possibly integrated into the chromosome. The 75-MDa plasmid could also apparently recombine with the 68-MDa plasmid, to which it was partially homologous.