The radular activity of dogwhelks, Nucella lapillus, drilling mussels, Mytilus edulis, was monitored by piezoelectric transducers and recorded by an automatic digital system. In this way, for the first time, the drilling behaviour of dogwhelks was analysed in detail. Radular activity was similar to that previously recorded for limpets, each radular stroke (rasp) being formed by a sequence of 1–8 unit events, each probably corresponding to the application of one row of radular teeth on the substratum. During drilling, radular rasping followed a consistent pattern, each bout of radular rasping (lasting about 6–8 min) being followed by a period of radular inactivity (lasting about 35 min), when chemical dissolution is probably applied. The periodicity with which rasping was applied decreased during the drilling sequence (from about 25 min in the first quarter of the drilling process to about 40 min in the last quarter), together with a decrease in the rasping rate, i.e. number of radular rasps per min (from about 12 to about five rasps per min). Accordingly, the total number of radular rasps recorded during the first quarter of the drilling time was higher than during the other quarters (from about 3000 to about 500 rasps). The role of the radula in shell penetration was therefore rather extensive, accounting, during the first quarter of the process, for about 34% of drilling time. The results are discussed in relation to studies of chemical dissolution and mussel shell ultrastructure.