ABSTRACT The time of activity and the duration of active periods (flight) of moths of the species Hyalophora cecropia has been determined by monitoring thoracic temperature. The metabolic cost of flight per day and per adult life has been determined directly by measuring O2 consumption and indirectly by analysis of cooling curves of individual moths. An energy balance sheet has been derived which gives the metabolic cost of flight and maintenance (during torpor) over the insect’ s adult life. The metabolic stores mobilized for daily activity appear to be fixed and independent of air temperature. This mobilization of fat stores may be under hormonal control. It is metabolically more expensive for moths to be active at low air temperatures. The number and duration of active periods at low air temperatures is reduced, but, the metabolic expenditure for activity is equal to that of animals held at higher air temperatures. Females have a smaller total energy reserve than males. The number of active periods per day is not significantly different between the sexes at any given temperature, but in females the active periods are significantly shorter in duration. The flight speed has been determined, and estimates of the flight range per day and per adult life have been calculated. The ecology of H. cecropia has been discussed with respect to the timing and duration of active periods, the range and speed of flight, and the energetic cost of flight and maintenance metabolism.