317 B. thuringiensis isolates were screened for activity against H. virescens larvae. Three isolates (HD-241, HD-244, and HD-263) were selected for further development and testing. In 1979 and 1980 all three isolates were compared to HD-1, the current isolate used commercially against lepidopterous pests, in small plot tests against H. virescens larvae on cotton. HD-263 was clearly the best isolate, so in 1981 and 1982 only HD-263 was used in conjunction with HD-1 in field tests against H. virescens on cotton. In all. but one test, HD-263 treated plots had numerically higher yields than did HD-1 treated plots. However, in only one year (1979) were the yield differences of seed cotton significantly different.