Levels of activity of a chymotrypsin-like enzyme in midguts of adult worker honeybees (Apis meillfera) from 2 hives were measured over a 3-year period. The principal between-year pattern was one of higher levels in winter than in either fall or spring. Considerable variation in enzymatic activity was found. Spearman rank correlation analysis of the chymotrypsin-like activity showed marked concordance in changes in level of activity between the 2 hives in each of the 2 years for which complete data were available, but between-year correlations for each hive were low (or negative) and varied widely. These results suggest that levels of chymotrypsin-like activity in the midgut of \vorker honeybees are markedly affected by exogenous, i.e. dietary, factors.