Two populations (S-1 and S-2) of the psocid, Liposcelis bostrychophila Badonnel were exposed to carbon dioxide enriched atmospheres. Carbon dioxide resistance developed at steady rates in these two populations during this study period. Selection with 35 and 55% CO(2) resulted in resistance development as expressed by LT(50). Resistance increased steadily under continuous selection to 4.6- and 5.3-fold by generation F(30) for S-1 and S-2, respectively. Throughout the selection process, the slopes of regression lines were always lower than that of the control. The results of biochemical assays showed that the activities of carboxyl esterase (CarE) and superoxide dismutase (SOD) in vitro increased in the selection process. Exposure to higher CO(2) content (HCC) resulted in a gradual decrease in CarE activity in both selected and control populations. Although the induction effect of CO(2) on SOD was brief, the induction times for the S-1 and S-2 were greater than those of the control. The elevated catalase (CAT) activity in association with resistance development was also evident, but no statistical correlation was found between CAT activity and HCC resistance. No significant differences were found in acid phosphatase and alkaline phosphatase activities in both selected and control populations during this study. This study demonstrated that high CarE and SOD activities were positively correlated to CO(2) resistance.