Crab predation may profoundly affect the structure of marine benthic mollusc populations and prey choice of crabs may be altered by organisms associated with their prey. We investigated effects of the shell-boring polychaete Polydora ciliata on shell strength of the periwinkle Littorina littorea, and the concomitant prey selectivity of one of its major predators, the crab Carcinus maenas. Shell strength of periwinkles measured as force required to cause cracking was significantly lower in snails infected with P. ciliata than those without infection. In laboratory predation experiments, C. maenas consumed more snails infected with P. ciliata than uninfected periwinkles in a given size class. This was true when infected and uninfected snails were offered independently and simultaneously. Although C. maenas preferred small-sized (13 to 17 mm shell height) over medium (18 to 21 mm) and large (22 to 24 mm) periwinkles, consumption of large snails with P. ciliata was twice as high as for medium-sized L. littorea without polychaetes. Thus, this shell-boring polychaete causes crabs to shift their prey choice, and may even eliminate a size refuge for large infected periwinkles. We conclude that P. ciliata modifies predator-prey interactions, and we propose more generally that a high prevalence of shell-colonising organisms may exert a strong indirect effect on the dynamics and size distributions of mollusc populations. © Inter-Research 2007.