The mussels Choromytilus meridionalis (Krauss) and Mytilus galloprovincialis (Lamarck) coexist with the predatory drilling whelks Nucella cingulata (Linnaeus), N. dubia (Krauss) and N. squamosa (Lamarck) on the west coast of southern Africa. Samples revealed that these whelks are trapped by byssus threads in intertidal mussel beds. Nucella of each species were enclosed with high and low densities of each mussel. Both mussels attached byssal threads to large numbers of all the Nucella species, and trapped some of those attacked. When M. galloprovincialis were enclosed with the non-drilling whelks Burnapena catarrhacta (Gmolin) and B. limbosa (Lamarck), the mussels attached few threads to the Burnapena, and did not trap any. C. meridionalis, which has weaker threads than M. galloprovincialis but produces them more rapidly, trapped Nucella more effectively when in dense clumps, and was most effective in trapping N. squamosa, which is the most common species of Nucella in beds of C. meridionalis. Small numbers of M. galloprovincialis trapped Nucella as effectively as dense clumps. M. galloprovincialis has strong byssus threads, and appears to aggregate rapidly into clumps. These features may have been important in its recent invasion of rocky shores in the region.