Abalone larvae settle (attach and metamorphose) in response to crustose coralline algae (CCA), but it is not known whether the settlement cues arise from the CCA or from microbes on their surface. The CCA Phymatolithon repandum induced close to 100% metamorphosis of Haliotis iris larvae. Pebbles and shells with a biofilm but without visible macroalgae induced, on average, 58% metamorphosis on H. iris larvae within 2 days, and biofilms are commonly used to cue settlement in H. iris hatcheries. This suggests that settlement on P. repandum might be triggered by the surface biofilm rather than the CCA itself. Treatment of P. repandum with antibiotics, germanium dioxide, and scrubbing did not significantly reduce settlement of H. iris larvae. Diatom strains isolated from P. repandum induced little attachment or metamorphosis at the low diatom densities (10³–10⁴ cells/cm²) found on P. repandum, but induced moderate attachment (30–85% after 4 days) and metamorphosis (2–40%) at high densities (5 × 10⁴-1 × 10⁶ cells/cm²). Nine bacterial strains isolated from P. repandum induced low-percent attachment (0–45% after 7 days) and metamorphosis (0–20%) if assayed in the presence of antibiotics. Parallel assays without antibiotics had higher attachment (40–80% after 7 days) and metamorphosis (5–50%), but this activity may have arisen from bacteria introduced with the larvae. Although bacteria and diatoms did induce some attachment and metamorphosis of H. iris larvae, they were less effective than P. repandum (95% within 2 days). Treatments that greatly reduced the biofilm on P. repandum had no effect on its settlement-inducing activity. This evidence supports the view of CCA as an especially potent source of inducers for temperate abalone larval settlement.