Brush border fragments were isolated from homogenates of mesenterons from the mosquito, Culex tarsalis, by a combination of Ca2+ precipitation and differential centrifugation. These preparations were routinely enriched seven-to eightfold for the brush border marker enzyme, leucine aminopeptidase. Alkaline phosphatase, a putative brush border marker for both vertebrate and invertebrate brush borders, was found to be unsuitable for Cx. tarsalis. lsoelectric focusing electrophoresis coupled with histochemical enzyme detection was used to enumerate isozymic species of nonspecific esterases [3], leucine aminopeptidase [I], and alkaline phosphatase [ I ] in isolated brush border fragments. Leucine aminopeptidase activity was solubilized by papain digestion, suggesting an extrinsic active site for this membrane-bound enzyme. The predominant nonspecific esterase isozyme remained membrane-bound. Conventional staining (ie, Coomassie Blue and silver) of proteins separated by isoelectric focusing, sodium dodecylsulfate, and two-dimensional electrophoresis indicated a simple pattern for brush border fragments, with two proteins predominating among the 11-14 routinely detected.