Abstract The proteasome plays a central role in ubiquitin-dependent and -independent proteolysis in eukaryotic cells, The hawkmoth proteasome was purified from larval body wall and characterized with respect to substrate specificity, sensitivity to protease inhibitors, and cross-reactivity with monoclonal antibodies (mAbs) raised against human placenta proteasome. Leupeptin selectively inhibited the trypsin-like activity (T-L) and N-ethylmaleimide inhibited both T-L and chymotrypsin-like activities, whereas 0.02% sodium dodecyl sulfate stimulated the peptidylglutamyl peptide hydrolase, branched-chain amino acid preferring, and caseinolytic activities 20-, 18-, and 3.8-fold, respectively. All four peptidase activities were inhibited by 3,4-dichloroisocoumarin. One-dimensional immunoblot analysis showed that the level and subunit composition of the proteasome varied between tissues, The relative levels of proteasome were high in intersegmental muscle and ovary, lower in Malpighian tubule, male accessory gland, and ventral nerve cord, and lowest in flight muscle and fat body, The tissues differed in the relative amount of a 41-kDa doublet; a 22-kDa subunit was present only in the male accessory gland, Two-dimensional polyacrylamide gel electrophoresis showed that the hawkmoth proteasome contained at least 26 subunits, compared with 28 subunits in lobster, Immunological analysis using four subunit-specific mAbs identified the putative homologs of the human zeta, C2, C3, and C8 α-type subunits in the hawkmoth and lobster enzymes, Two of the four mAbs reacted with three or more of the hawkmoth subunits and three of the mAbs reacted with two or more of the lobster subunits. In addition, two other mAbs that recognize epitopes shared by a number of α-type subunits indicated that at least 15 (lobster) or 16 (hawkmoth) subunits were α-type. These results suggest that much of the subunit complexity of the arthropod proteasomes is a consequence of extensive post-translational modifications.