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Dipeptidase and carboxypeptidase A activities were determined in cells and luminal contents of the fore-, mid-, and hind-midgut of Musca dornestica larvae. Dipeptidase activity was found mainly in hind-midgut cells, whereas carboxypeptidase activity was recovered in major amounts in both cells and in luminal contents of hind-midguts. The subcellular distribution of dipeptidase and part of the carboxypeptidase A activities is similar to that of a plasma membrane enzyme marker (aminopeptidase), suggesting that these activities are bound to the microvillar membranes. Soluble carboxypeptidase A seems to occur both bound to secretory vesicles and trapped in the cell glycocalyx.Based o n density-gradient ultracentrifugation and thermal inactivation, there seems to be only one molecular species of each of the following enzymes (soluble in water or solubilized in Triton X-100): membrane-bound dipeptidase

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