A borehole analysis was performed on valves of Mactra isabelleana (d’Orbigny, 1846) and Glycymeris longior (Sowerby, 1833) from the marine Quaternary of San Clemente del Tuyú, in the province of Buenos Aires, (Argentina). The study suggested an intensive predation on both species. Glycymeris longior was the preferred prey, although the amount of bored valves of Mactra isabelleana is larger, because the latter is far more abundant in the quaternary deposits. The most frequent borehole location in M. isabelleana is the postumbonal area whereas in G. longior it is the central area. A high correlation between size of predation (diameter of the borehole) and prey length was observed for both species. Mactra isabelleana was preyed at the most abundant sizes. The greatest amount of preyed valves of M. isabelleana coincides with the most abundant sizes. Glycymeris longior was preyed more frequently at 13-15 mm and the most abundant sizes were 11-13 mm. The borehole morphology and behavioral features suggest that the predator belongs to Naticidae.