Octopus dofleini (Wulker) drills holes in mollusc shells, enabling it to introduce venom. Boreholes in the bivalve Saxidomus giganteus (Deshayes) were generally irregular. Thick shells were more likely to have incomplete boreholes, and the diameter of completed boreholes was larger than in thin shells. A comparison of boreholes to fractured and artificially drilled surfaces suggests that O. dofleini employs chemical dissolution of the shell during drilling.