Incubation of Fascioloides magna cercariae in catechol produced tanning of the cystogenous granules, indicating the presence of phenolase. Together with observations on the natural coloration of the metacercarial cyst, this experimentally induced tanning suggests that the natural color of the fasciolid metacercarial cyst is due to a quinone tanning process, analogous to that occurring in the fasciolid egg-shell.