The potential activity of 65 strains of the biopesticide Bacillus thuringiensis was evaluated against the first instar larvae of the potato tuber moth Phthorimaea operculella. The larvae of this insect pest showed to be susceptible to some varieties kurstaki, thuringiensis, tolivorthi, gulleriue, kenyac. and aizarvai. Data on the LC,, and potencies of the highly active varieties (causing larval death of 80% or more) indicate that var. galleriae was the most potent. The LC,,, for other varieties, ranged between 28.83-101 pg/ml. The different larval instars of P. operculellu were affected by B. thuringiensis var. galleriae HD-234 in varying degrees, showing a gradual increase of LC,, with the increase of larval age. U S