Abstract The parasporal body of the mosquitocidal isolate (PG-14) of Bacillus thuringiensis subsp. norrisoni (BTM) contains five major proteins with molecular masses of, respectively, 27.3, 65, 128, 135, and 144 kDa. Protein corresponding in mass to the first four of these also occur in the mosquitocidal strain, B. thuringiensis subsp. israelensis (BTI), and it is thought therefore that the mosquitocidal activity of both strains is due to these four proteins. In other studies it has been shown that each of these proteins exhibits from moderate to high toxicity to mosquitoes, though the specific toxicity of the 144 kDa protein in PG-14 to mosquitoes remains unknown. In the present study, two parasporal body mutants (M146 and M242) of PG-14 were developed growing the wild-type strain at 42°C. The parasporal body of M146 contained less of the 65-kDa protein and was less toxic (LC 50 = 108 ng/ml) to mosquitoes than the wild-type strain (LC 50 = 8.3 ng/ml). The parasporal body of M242 consisted of a bipyramidal crystal composed of a 144-kDa protein that was not toxic to the mosquito, Aedes aegypti, but exhibited substantial toxicity (LC 50 = 2.5 μg/ml) to the lepidopteran, Trichoplusia ni. Because the parasporal bodies of BTI and BTM PG-14 are similar in mosquitocidal toxicity on a weight basis, the latter results suggest the 144-kDa protein, though not mosquitocidal alone, can contribute to mosquitocidal, activity when in the presence of other mosquitocidal proteins.