A standardized laboratory bioassay was used to quantify the lethal and sublethal responsesof larval and adult cottonwood leaf beetles,Chrysomela scriptaF., toBacillus thuringiensis var. san diego, formulated as M-One standard powder (Mycogen Corporation, San Diego). The median lethal concentration (LC50) for second instars, after a 96-h exposure toB. thuringiensis var. san diego, was 21,000 Colorado potato beetle international units per milliliter water. The LC50estimates for the third instars and adults were 20 and 40 times higher than for second instars, respectively. Larval LC50estimates were halved when mortality for the entire larval period was included in the LC50estimate. Adult mortality continued for approximately 14 d after initial exposure toB. thuringiensis var. san diego. The adult LC50calculated at 14 d was half the 4-d estimate. Age of adults at treatment did not significantly affect the LC50. Median lethal times (LT50) were similar for larvae and adults, with overlapping confidence limits ranging from 2.1 to 3.5 d. Larvae surviving treatments as second and third instars showed a significant dose-dependent decrease in adult dry weight at eclosion and an increase in the larval developmental period.