The body of Trichuris suis females maintained in vitro under a gas phase of 95% air 5% CO2 develops a brown pigment that is apparent after 1 day and intensifies with time. Development of the brown pigment is prevented by maintaining the parasites in an anaerobic gas phase (95% N2, 5% CO2), but tanning commences when worms are returned to aerobic conditions. Tanning was not observed in males. Intact female T. suis take up oxygen at a considerably higher rate than males. Supernatant fractions (10,000 g) and pellets from whole worm homogenates of females both convert dihydroxyphenylalanine (DOPA) to dopachrome, suggesting the presence of a phenol oxidase. About 70% of the total phenol oxidase activity in females was in the pellet and about 30% in the supernatant fraction. Homogenates of male worms contained minimal phenol oxidase activity. Polarographic assay of phenol oxidase activity confirmed the presence of this enzyme in female T. suis. Female homogenates oxidized both dihydroxyphenylalanine and 4-methylcatechol and to a lesser extent hydroquinone. This oxidation was inhibited (> 90%) by diethyldithiocarbamate. Males did not oxidize any of the substrates tested. These results suggest that an enzyme of the phenol oxidase type is present in female worms but is probably inactive because of low oxygen tensions in the swine colon. The function of this enzyme in T. suis is unknown but is most likely associated with tanning of eggshell proteins or other aspects of eggshell synthesis.