Carbaryl toxicity, metabolism, and distribution in adults of three species of grass bugs _from the genus Labops were studied in relation to species, sex, and treatment with piperonyl butoxide. Lc50 values for 8 hour exposure periods ranged from 0.02-0.14, 0.03-0.3, and 0.2-0.7 ��g carbaryl/vial for L. utahensis, L. hirtus, and L. hesperius respectively. The males were more susceptible to carbaryl than females. Males of L. utahensis were more susceptible than L. hirtus and L. hesperius respectively. The synergist difference values (Lc50 of carbaryl alone - Lc50 values of carbaryl after piperonyl butoxide treatment) were measured. The percent dependency of these _insects on MFOs for detoxifying carbaryl was estimated based on the theoretical synergist difference which was calculated by the equation Log LC50 = 1.014 log SD - 0.009. The percent dependency values were 38-59, 25-46, and 13-33% for L. hesperius, L. hirtus, and L. utahensis, respectively. Males of L. utahensis had the lowest percent dependency upon MFOs in detoxifying carbaryl (13%) indicating the possibility that carbaryl toxicity may be controlled by other potential defense mechanisms which are relatively ineffective themselves in view of the low tolerances of the insects to carbaryl. Treatment with piperonyl butoxide resulted in greater enhancement of carbaryl toxicity against L. hesperius (synergized Lc50 0.1-0.26 ��g carbaryl/vial) while it showed a moderate effect on L. hirtus (synergized Lc50 0.02-0.16 ��g carbaryl/vial). Piperonyl butoxide's effect was less pronounced in the case of L. utahensis (synergized Lc50 0.013- 0.09 ��g carbaryl/vial. Unmetabolized carbaryl was the principle compound isolated from the bugs after 6 hours from treatment, being more prominent in males of L. hirtus (71.1% of the total metabolites) and less prominent in females hesperius (36.7% of the total metabolites). The mechanism of detoxication appeared to include ring hydroxylation for both species and sexes. 4 and 5-hydroxycarbaryl were the only metabolites associated with the degrading of carbaryl by the bugs, since the levels of metabolites obtained were too low for accurate quantitation. Pretreatment with piperonyl butoxide prevented the appearance of both carbaryl metabolites in the organosoluble fraction and increased the accumulation of unmetabolized carbaryl. This effect was probably due to inhibition of the insect's MFO system. Generally, this study showed a good correlation between the bioassays and the metabolic studies, thus reflecting the effectiveness of the bioassays along with synergist difference (SD) and percent dependency concepts in establishing some conclusions regarding the MFOs of Labops bugs. Further application of these techniques with agricultural insects should provide a practical means of characterizing field populations for insecticide tolerance, relative levels of MFOs and their role as a defense mechanism.