The genes of the bacterial sialidases from Clostridium sordellii G12, C. perfringens A99, Salmonella typhimurium LT-2 and Vibrio cholerae 395 sequenced so far were examined for homologies and were compared with sequences of viral sialidases. Each of the bacterial sialidases contains a short sequence of twelve amino-acids, which is repeated at four positions in the protein. All these sequences exhibit significant similarities. Comparing the repeated sequences of the four sialidases, five amino-acids were found to be highly conserved at defined positions: Ser-X-Asp-X-Gly-X-Thr-Trp. Additionally, most of the distances between the four repeated regions are also conserved among the different sialidases. The conserved bacterial sequences show similarity with sialidases of influenza A H7N1 and H13N9.