Differential scanning calorimetry and microrespiration were used to determine the effects of the biopesticide Bacillius thuringensis (Bt) toxin on the metabolism of Pandemis leafroller, Pandemis purusana (Kearfott). The metabolic heat rate, CO₂ evolution and O₂ consumption of 2^nd and 3^rd instars following a 2 h exposure to different concentrations of Bt toxin administered through artificial diet were measured from day 0 to 3. Survival (mortality) was recorded from day 0 to 4. The metabolic heat rates of the Bt challenged larvae were significantly lower than those of the untreated controls. Whereas CO₂ evolution and O₂ consumption were not significantly different between the treated and controls, except for larvae challenged at the highest dose of Bt on day 2. However, the calorespiromic ratios, kJ/mole O₂ and kJ/mole CO₂, of all Bt challenged larvae were significantly lower than untreated controls. The respiratory quotient CO₂/O₂ was lowest for larvae challenged at the two highest doses. Mortality of the larvae followed a normal pattern, with the highest mortality at the highest doses and lowest at the lowest doses. Differential scanning calorimetry can be useful in identifying larvae with compromised metabolism as a result of Bt exposure.