ABSTRACT: Larval settlement is critical to the establishment and maintenance of marine communities and is influenced by exogenous (bio-physical) and endogenous (behavioural or physiological) factors. Consequently, an understanding of settlement behaviour is specific to the bio-physical conditions and/or ontogenetic stage(s) examined. For intertidal organisms, the presence of settlement-inducing or -inhibiting cues is well known, including various effects of biofilm and conspecifics. Studies of marine mussels, however, have incorporated little ontogenetic resolution and have almost exclusively been limited to investigation of larval–adult interactions. The present work examines behavioural responses of primary and secondary settlers of indigenous (Perna perna) and invasive (Mytilus galloprovincialis) mussels to biofilm and settler presence on artificial collectors at 2 sites in South Africa. Specifically testing for larva–settler and settler–settler interactions, this approach allowed for replication of site and species. It was hypothesised that biofilm and conspecific settlers would increase primary and secondary settlement of both species alike. Settler densities at one site were too low to detect treatment effects. At the other site, however, primary settlers of both species were significantly attracted to filmed rather than bare collectors (ANOVA, P. perna, p < 0.01; M. galloprovincialis, p < 0.05) and also avoided conspecific settlers, while secondary P. perna settlers preferred the combination of biofilm and conspecifics to other treatments (ANOVA, p < 0.05). The findings represent an important ontogenetic shift in settlement behaviour with broad methodological and ecological implications, including possible explanations for natural patterns of habitat use by primary settlers, and the inhibition of switching between alternative stable states.