Relative attractiveness of Crassostrea virginica and Modiolus demissus to Urosalpinx was studied to determine whether oxygen consumption of prey is correlated with selection by the predator. Comparisons between individuals of one prey species and both species demonstrated a direct correlation between oxygen consumption (used as an index of respiratory metabolism) and number of Urosalpinx attracted. Four factors affecting metabolic rate of prey were studied: species, age, growth rate, and feeding. In each case the more attractive animals had a proportionately higher respiratory rate. Urosalpinx responded in equal numbers to groups of prey of different individual, but equal group, respiratory rates. Thus this work supports the hypothesis that a metabolic product mediates the predator’s choice of a particular individual within a group of prey. Selection is independent of concentration of dissolved oxygen or carbon dioxide.