A recent hypothesis in the zoopIankton literature states that zooplankton acclimate to ambient food concentra~ tions such that higher digestive enzyme activities and, consequently, higher maximum ingestion rates are achieved at higher food levels. To test this hypothesis, adult female Calanus pacificus, collected from the main basin of Puget Sound, Washington, USA, in August 1979 and May 1982, were conditioned for 2wk at different concentrations of the diatom Thalassiosira weissflogii (=fluviatilis). Ingestion rates and the activity of the digestive enzymes laminari~ase, maltase, and cellobiase were measured periodically during acclimation and in a block-designed feeding experiment at the end of acclimation. Consistent with the hypothesis, maximum ingestion rate and digestive enzyme activity were positively correlated. However, in contrast to the hypothesized mecha nism, this result arose because both maximum ingestion rate and digestive enzyme activity were negatively cor related with food concentration during acclimation. The enhanced ingestion of copepods following long-term (12 to [4 d) acclimation to low food is similar to that previously described for short-term (e.g. t d) starvation. It might be energetically optimal for copepods experiencing a patchy food environment to maintain higher levels of digestive enzymes at low food concentrations in order to exploit high concentrations of food when encountered.