Daily time and energy budgets were compared for two sympatric populations of sea hares in Hawaii: the nocturnal Aplysia dactylomela and the diurnal A. parvula. Time budgets were determined from direct observation of field animals and equivalent energy costs for each activity observed were determined from oxygen uptake ( V O 2 ) measurements on animals engaged in the same activity in a laboratory respirometer. Activities measured for A. dactylomela included feeding, copulating, resting, moving, egg-laying and being buried and for A. Parvula included feeding and resting, with estimates being made (based on the data for A. dactylomela) for the other activities not recorded in the respirometer. Least energy-demanding activities for A. dactylomela were resting, copulating, being buried and egg-laying (18.8, 18.8, 15.4 and 21.4 J ·20-g ind −1·h −1, respectively). Least energy-demanding activity for A. parvula was resting (0.53 J · 0.6-g ind −1 · h −1). Most costly activities for A. dactylomela were grazing and moving (39.4 and 42.4 J · 20-g ind −1 · h −1) and for A. parvula was grazing ( 1.13 J · 0.6-g ind −1 · h −1; moving costs were not determined for this species). Overall, grazing increased energy expenditure by ≈210% over resting levels in the two species and locomotion by 225% in A. dactylomela. The most energy-demanding period of the day for A. dactylomela was just prior to dawn when feeding was most intense in preparation for 12 h of resting and digestion during the daylight period. The most energy-demanding period of the day for A. parvula was in the afternoon when most animals were feeding in preparation for a nighttime period of rest. In both species, allocation of time and energy to copulation were highest immediately after rest, following digestion of a full crop of food. At this time, the animals were active, blood-sugar levels presumably high and pursuit of mates intense.