The epiphytic bryozoan Membranipora membranacea encrusts the surface of kelp blades, causing recurrent large-scale defoliation events in kelp beds off the Atlantic coast of Nova Scotia, Canada. The gastropod Lacuna vincta grazes kelp, creating perforations that weaken blade tissues and increase the fragmentation rate. We assess the interaction between M. membranacea and L. vincta by measuring the grazing rate of snails on bryozoan-encrusted and non-encrusted kelp (Saccharina latissima) in no-choice and choice experiments in the laboratory conducted in November and December 2010. There was no effect of diet on grazing rate in no-choice experiments. In choice experiments, however, L. vincta grazed significantly more non-encrusted than encrusted kelp (7.1 versus 1.1 mg snail−1 d−1), and grazing rate of non-encrusted kelp was almost twice that in the no-choice experiment (3.8 mg snail−1 d−1), indicating that snails may avoid colonies of M. membranacea on partially encrusted kelp blades. We found no effect of diet on growth, reproduction and survival of snails maintained for four weeks on encrusted or non-encrusted kelp. By concentrating grazing damage on non-encrusted areas of blades, L. vincta may act synergistically with M. membranacea to increase the likelihood of blade breakage and canopy loss. This indirect effect of the invasive bryozoan could augment its direct effect on the standing biomass of native kelp beds and detrital export to adjacent communities.