Cytochrome P450 monooxygenase activities towards aldrin and 7-ethoxycoumarin were detected in microsomes prepared from L1 and L3 larvae of Haemonchus contortus, and very low levels of activity towards aldrin were detected in adults. Larval activities were NADPH-dependent, suppressed by carbon monoxide (CO) and piperonyl butoxide, and induced (up to 60-fold) by exposure to phenobarbital. Different patterns of expression of activities towards the 2 substrates in various life-stages, as well as different sensitivities to piperonyl butoxide, suggested the presence of more than 1 cytochrome P450 enzyme. Cytochrome P450 itself could only be detected by spectral assay in phenobarbital-treated L3 larvae. It is most likely that the observed shift of several nanometres in the position of the spectral peak of cytochrome P450 was due to the presence of other CO-reactive haemoproteins (probably cytochrome oxidases). It is apparent that H. contortus possesses cytochrome P450 monooxygenase activities; however, they may be important only in the free-living stages, and be of little significance in parasitic stages existing in oxygen-poor environments.