In an earlier study (Corner et al. 1974) the copepod Calanus helgolandicus (Claus) was found to feed actively on nauplii of the barnacle Elminius modestus Darwin, one adult female ingesting a maximum daily ration equivalent to 25% of the body nitrogen. The present work, in which Elminius nauplii have again been used as a diet, extends the earlier study by determining how efficiently Calanus digests an animal food. In addition, the methods used have provided information on other aspects of carnivorous feeding by Calanus, including the relationship between food concentration and size of ration ingested as well as gross growth efficiency, i.e. the proportion of ingested food invested in growth.