The completely plated morph in the threespine stickleback (Gasterosteus aculeatus) is considered to be the ancestral plate condition and is the predominant morph inhabiting marine waters and numerous northern freshwaters. Evolutionary aspects of this distribution have been widely addressed yet functional mechanisms remain obscure. Experiments described here using the common piscivore (Oncorhynchus clarki) show that the posterior plates of G. aculeatus interfere with the swallowing abilities of the piscivore, possibly by disrupting pharyngeal jaw retraction, and this leads to increased escape opportunities of the stickleback. The advantage of the completely plated condition is most expressed at higher ratios of prey diameter to predator mouth diameter and appears to have a defensive effect comparable to that of dorsal and pelvic spines. This attribute, combined with the physical protection that plates offer against puncturing, would be particularly beneficial where there is high probability of capture by toothed predators. Such a selection regime appears to characterize the predominantly limnetic and pelagic habitats where marine stickleback are found and may account for the wide geographical distribution of the completely plated morph and its persistence from the Miocene.