We quantified variation in performance under greenhouse conditionsamong seven populations of Scirpophaga incertulas (Walker)from Luzon Island, Philippines, on three rice varieties: ‘IR58’transformed with the cry1Ab gene from Bacillusthuringiensis Berliner, and nontransgenic IR58 and IR62. On IR62,S. incertulas performance did not differ among provinces forany of the 10 parameters measured, but there was a significant effectof town within province for one parameter, 20-d-old larval weight.Larval survival after 48 h on cry1Ab-transformed IR58did not differ significantly among provinces, but did differsignificantly among towns within a province. There was no geographicvariation in larval survival after 48 h on control plants of IR58.Surviving insects from the cry1Ab-transformed IR58 weretransferred to IR62 to complete development. There was no geographicvariation in the percentage of insects completing development to adultemergence and the time required by the transferred female insects tocomplete development. However, there was variation among provinces inmale developmental time. The absence of geographic variation onnontransgenic IR58 and the very limited variation on IR62 indicatedthat there was little variation in general vigor among the S.incertulas populations and thus that the variation in performanceon cry1Ab-transformed IR58 was probably attributable todifferences in susceptibility to Cry1Ab.