Laboratory experiments were conducted to identify settlement cues of larval top shell Turbo cornutus on the articulated coralline alga Marginisporum crassissima. Fragments of M. crassissima strongly induced the larval settlement of the top shell. The settlement percentage of the larvae on ethanol-killed fragments of M. crassissima was significantly lower than on intact algal fragments, even though algal morphology did not differ between live and dead fragments. When the densities of diatoms on algal surfaces were altered by soaking the algal fragments in a germanium dioxide solution or a modified Jørgensen medium, neither treatment (increased or reduced diatom densities) affected the larval settlement percentage on M. crassissima. Similarly, reducing bacterial densities with antibiotics did not affect the larval settlement response to the treated M. crassissima. Top shell larvae were induced to settle by crude extracts of crushed M. crassissima but not by seawater where fragments of M. crassissima were immersed. These results suggest that the main cues inducing larval settlement of T. cornutus are chemical compounds derived from the alga, which have poor water solubility; algal morphology, and surface biofilms did not directly contribute to settlement by top shell larvae.