Crithidia deanei is an insect trypanosomatid that presents bacterial endosymbionts. In this study, the expression of surface gp63-like proteins in the endosymbiont-bearing and aposymbiotic strains of C. deanei was compared through flow cytometry analysis using anti-gp63 antibodies raised against recombinant gp63 from Leishmania mexicana. The absence of the endosymbiont reduced the binding of anti-gp63 to the cell surface, suggesting a regulation of this metallo-type enzyme by the presence of the endosymbiont. For fluorescence-activated cell sorter (FACS) analyses 1.0 “ 10⁶ cells were incubated for 1 h at room temperature with either a 1:1000 dilution of rabbit anti-gp63 antiserum or rabbit pre-immune serum and then incubated for an additional hour with a 1:250 dilution of fluorescein isothiocyanate (FITC)-labeled goat anti-rabbit IgG. Immunoblots analysis was performed with the supernatants from the reaction mixtures of either Bacillus thuringiensis phospholipase C (PLC)-treated or control cells (equivalent to 5.0 “ 10⁶ cells). The primary antibodies used were a rabbit antiserum raised against recombinant gp63 (H50) from Leishmania mexicana at 1:500 dilution or anti-cross-reacting determinant (CRD) and the secondary antibody used was horseradish peroxidase-conjugated goat anti-rabbit IgG at 1:25000. Flow cytometry and Western blotting analyses indicated that these molecules were glycosylphosphatidylinositol (GPI)-anchored to the surface. Live parasites treated or not with PLC (2.0 “ 10⁶ cells in 100 ml) were added to dissected guts of adult female mosquitoes (Aedes aegypti), which were sliced open longitudinally (10/group) and incubated for 1 h at room temperature in PBS. The aposymbiotic strain of C. deanei presented interaction rates about 2-fold lower with guts. It has been established that gp63 homologues are relevant to the adhesion of several lower trypanosomatids, including C. deanei, to the insect gut. Parasites (5.0 “ 10⁷ cells) were incubated with 0.1 U/ml PLC in 50 ml of phosphate buffered saline (PBS) supplemented with 20% glycerol for 3 h at 37°C. The treatment of the wild strain with PLC reduced the exposition of surface gp63 and the adhesion rate to A. aegypti guts to similar levels to those found in the untreated cured strain. Collectively, these results suggest that the cell surface gp63 homologues may account for the better interaction of the wild strain of C. deanei to the insect gut epithelial cells.