Esterase activity of the gypsy moth, Lymantria dispar (L.), was studied by a spectrophotometric method using 1200 g supernatants of mid-fifth instar larval tissues as the enzyme source and 1-naphthyl acetate as substrate. Using midgut preparation, the K m was determined to be 4.25 × 10 −5 M 1-naphthyl acetate, and the V max 942 nmoles mg −1 min −1 at 30°C. The hydrolysis rate was linear for the first 25–30 min. Enhancement in esterase activity was apparent up to 50°C. The optimum pH was between 7.5–7.7. The specific activities (nmoles mg −1 min −1 at 30°C) of tissues were as follows: midgut (1004), Malpighian tubules (334), nerve cord (125), fat body (111), hindgut (108), foregut (84), gonads (83), muscles (55), brain (43), integument (20), and haemolymph (7). Midgut accounted for 91.5% of total esterase activity of all tissues. With the exception of brain and nerve cord where acetylcholinesterase (EC 3.1.1.7) showed higher activity, all other tissues revealed carboxylesterase (EC 3.1.1.1.) as the predominant enzyme. There was evidence of some arylesterase (EC 3.1.1.2.), particularly in the hindgut, nerve cord, and the brain. Midgut was the tissue richest in esterase actvity, which was largely carboxylesterase activity. Its titre displayed a cyclic pattern that correlated with feeding activity and larval development.