The effects of different types of substrate and of primary biofilm formed on them in relation to their capacity to attract settlement of competent larvae of the northern scallop Argopecten purpuratus, was studied. The hypothesis that different substrates permit the formation of different biofilm over time, which affect the intensity of scallop larval settlement on each different substrate, was tested. Substrates including Netlon @ fly screen, scallop shell, and asbestos-cement were incubated in 50 μm-filtered flowing sea water to allow formation of primary biofilm for periods of 6, 12, and 18 days. Each substrate, fouled for the time periods listed, was exposed to 200-300 competent scallop larvae in test volumes of 300 mL filtered seawater for periods of 72 h at ambient temperature (15.3-18.6°C), after which the numbers of larvae settled on each substrate were determined. The results showed that the type of substrate influenced the intensity of scallop larval settlement. The highest settlement occurred on the asbestos-cement (29.6-62.3%); intermediate levels of settlement were observed on the fly screen (17.1-21.2%) and scallop shell (13.0-30.0%), and the lowest levels occurred on the Netlon (4.3-15.2%). A direct relationship was also observed between the percentage of larval settlement and the age of the biofilm, except for asbestos-cement. The type of substrate and age of the biofilm showed a synergistic effect on the intensity of larval settlement.