Adult populations of the psocid, Liposcelis bostrychophila, were exposed for 30 generations to 35% CO2, 1% O2, and 64% N2 at 28℃, 80% RH, in order to select a strain resistant to controlled atmosphere (CA). Selection pressure was maintained at around 70% mortality. At the 30th generation, comparison of sensitivity between the selected strain and the original susceptible strain (CA-S) indicated a tolerance factor (TF) at the 50% mortality level (LT50) of 5.6-fold. Throughout the selection process, log-time against probit-mortality lines remained roughly parallel and the slopes remained lower than that of CA-S until the last generation. Implications are that at high levels of selection, multiple genetic factors continued to act together even at the 30th generation to select for adaptation to survival in a CA environment. Removal of selection pressure for five generations from one subpopulation of the selected strains from the 25th generation caused significant reduction in resistance. In the absence of CA exposure, the selected strain possessed the fitness defect. The CA-selected strain was calculated by R0 to have a fitness value of 0.39 relative to CA-S. In addition, the potential effective measures in the slowdown of resistance to CA of stored-product insects are also discussed.