The present study was aimed to determine the toxic effect of phosphine on microsomal enzymes (NADPH-Cytochrome P450 reductase (NADPH-CPR), ethylmorphine-N demethylase and aniline 4-hydroxylase) and soluble protein contents of a wheat grain pest, Trogoderma granarium collected from godowns of Punjab, Pakistan. Six populations of khpara beetle with different levels of susceptibility to phosphine were used in this study. Based on LC50, five populations (previously exposed to phosphine for 15 years)viz., Mandi Bahauddin-I (MBDIN-I), Mandi Bahauddin-II (MBDIN-II), Gujrat, Gujranwala and Sargodha were labeled as phosphine-tolerant populations and one population never exposed to any insecticide/fumigant for 13 years was termed as phosphine-susceptible population. Percent change in the activities of above mentioned microsomal enzymes and soluble protein contents in 4th and 6th instar larvae and adult beetles of phosphine-tolerant populations were evaluated with reference to phosphine-susceptible population as a control. The activities of NADPH-CPR, ethylmorphine-N demethylase, aniline 4-hydroxylase and soluble protein contents were significantly increased in all field collected phosphine-tolerant populations with reference to phosphine-susceptible population. Among developmental stages, the 4th instar larvae possessed higher microsomal enzyme activities than 6th instar larvae and adult beetles in all populations. The increased level of microsomal enzymes in phosphine-tolerant populations as compared to susceptible population has pointed some correlation between microsomal enzymes and phosphine tolerance.