The members of the Pacifastin family are serine protease inhibitors found in insects and crustacean. They are either small inhibitors (made of one consensus cysteine-rich motif) or proteins (4-9 motifs). Some of these inhibitors are characterized by a species selectivity for the trypsin inhibition. Structural data discriminate the small inhibitors that apparently look very similar into two groups. Interestingly, the inhibitors that display species selectivity fall in the same structural group.