Drosophila can provide valuable insights into the structural and molecular basis of egg fertilization in insects, an issue practically unexplored, and the conservation of mechanisms of gamete interactions present in other organisms. The plasma membrane of D. melanogaster spermatozoa contains four intrinsic glycoproteins with glycosidase activity that we have purified and fully characterized: two β-N-acetylhexosaminidases, DmHEXA and DmHEXB, an α-mannosidase and an  α-L-fucosidase (Cattaneo, F. et al. 2002. Insect Biochem. Mol. Biol. 32: 929-941). HEXA, HEXB and α-mannosidase are present on spermatozoa throughout the genus Drosophila, whereas α-L-fucosidase is restricted to the melanogaster group. Glycoproteins with terminal sugars complementary to the enzymes of the sperm surface are present on the egg coats, at the site of sperm entry. Thus, it is conceivable that, like in other model organisms, also in Drosophila the sperm surface glycosidases mediate sperm-egg recognition. Whereas for α-mannosidase and α-L-fucosidase this role is still presumptive, we demonstrated previously that HEXA and HEXB are actually essential for sperm penetration into the egg (Perotti, M.E. et al. 2001. Mol. Reprod. Dev. 60: 248-259). We have therefore focused our interest on these enzymes. We identified three putative D. melanogaster genes predicted to code for β-N-acetylhexosaminidases, Hexo1, Hexo2 and fdl. All the genes are expressed in the male germ line. fdl codes for a homolog of the α-subunit of the mammalian Hex A. Hexo1 and Hexo2 encode two homologs of the β-subunit of all known β-N-acetylhexosaminidases, which we have named β₁ and β₂, respectively. Immunochemistry demonstrated that fdl and Hexo2 products combine to give rise to the heterodimeric DmHEXA (α/β₂), whereas Hexo1 and Hexo2 products form the heterodimeric DmHEXB (β₁/β₂). Immunocitochemistry showed that the gene products localize to distinct domains of the sperm plasma membrane. The presence of DmHEXA over the acrosome suggests a major role for this enzyme in fertilization.