The potential bacterial mutagenicity of the beta-exotoxin ofBacillus thuringiensisvar.thuringiensiswas detennined by theSalmollella-microsomalassay (Ames test) and found to be nonmutagenic. The activity of three experimental preparations ofB. thuringiensisvar.thuringiensiswere determined by feeding the preparations to house fly larvae and to neonateColorado potato beetle larvae,Leplinotarsa decemlineala(Say). Comparisons with controls were made as to food consumption and mortality rates, and the LDIOthrough LDQ, were calculated. In field tests of 0.05-ha plots of potatoes, an experimental preparation ofB. lhuringiensisvar.lhurillgiellsisprovided excellent control of the Colorado potato beetle when applied weekly at the rate of 1,892ml/O.4ha.