Abstract: At the present paper,adult populations of the psocid,Liposcelis bostrychophila,were exposed respectively for 30 generations to two atmospheres containing 0.5% and 1% O2(N2 in mixture as balance),in order to select strains resistant to low O2 content (LOC) atmosphere.Selection pressure was maintained at around 70% mortality.At the 30th generation,comparison of sensitivity between the selected strains (LOC1 and LOC2) and the original susceptible strain (CA-S) indicated a tolerance factor (TF) at the 50% mortality level (LT50) of 4.7- and 3.9-fold,respectively.Throughout the selection process,log-time against probit-mortality lines remained roughly parallel and the slopes remained lower than that of CA-S strain until the last generation.The implication is that at high level of selection,multiple genetic factors continued act together even at the 30th generation to select for adaptation to survival at depleted O2 concentration.Up to 30th generation,two selected strains still possessed the genetic potential to develop resistance to LOC.Removal of selection pressure for 5 generations from 2 sub-populations of two selected strains from 25th generation caused significant reduction in resistance.In the absence of CA exposure,the two selected strains all possessed the reproductive disadvantages or fitness defect.LOC1 and LOC2 were calculated by R0 to have a fitness value of 0.56 and 0.75 relative to unselected strain,respectively.