We examined the role of substrate interactions and the biological activity of cultured shellfish on the settlement and growth of fouling macroalgae and the toxic epiphyte Prorocentrum lima. Six miniature live-mussel (Mytilus edulis) socks (30 cm long) and 6 dummy socks (created with empty mussel valves) were hung along a horizontal long-line at a sheltered coastal site in Nova Scotia, Canada. After 4 and 9 wk, 3 socks of each treatment were harvested and the biomass of fouling macroalgae and concentration of P. lima cells were determined. Macroalgal fouling, which was almost entirely Pilayella littoralis, was greater on the live-mussel socks than on the dummy socks (ANOVA, p < 0.01) after 9 wk. Densities of P. lima cells g-1 dry wt of fouling biomass were higher on the dummy socks than the live-mussel socks (ANOVA, p < 0.01) during the entire experiment. Liquid chromatography-mass spectrometry (LC-MS) analysis showed the epiphytic material to contain the diarrhetic shellfish toxins dinophysistoxin-1 (DTX1) and okadaic acid (OA), which had been previously identified in culture mussels from this site. The data suggest that mussel culture provides a microenvironment that favours the colonization and growth of macroalgae, but the growth and colonization densities of the epiphytic dinoflagellate are not directly dependent on that fouling biomass.