The proportion of bivalve shells with drill holes in fossil assemblages is commonly used as a measure of the intensity of predation by drilling gastropods. Previous studies have assumed that drilled and undrilled bivalve shells have equal preservation potentials. This assumption has been tested by measuring the mechanical strength of drilled and undrilled valves of the Recent bivalve Mulinia lateralis. Under compressive loads, drilled valves are significantly weaker than undrilled valves. The results suggest that drilled valves may break preferentially and hence inferred patterns of predation may reflect taphonomic as well as biological processes