Abstract Raised levels of carbon dioxide (CO 2) are known to be toxic to many insect species. Sitophilus oryzae had previously been shown to be one of the more tolerant species to this treatment. Cohorts of known age S. oryzae were treated at 25 °C and 60% r.h. with CO 2 concentrations in the range of 15 to 100% with air as the balance. The nature of the mortality data obtained precluded the use of conventional probit analysis. Fitting the logit of response against a cubic polynomial of time gave a satisfactory fit for most data. Confidence intervals were estimated using a series of best cubic fits around tentative values of LT x. It was not possible to define a single mortality response surface taking into consideration concentration and time. Therefore, individual LT x's and confidence intervals were calculated for each concentration and age cohort. At higher CO 2 concentrations pupae were the most tolerant (LT 99 = 6.9 d in 65% CO 2) and adults (LT 99 = 1.5 d in 65% CO 2) the most susceptible stages. At lower concentrations eggs were most susceptible (LT 99 = 8.5 d in 20% CO 2) with all other stages having an LT 99 of longer than 45 d.