ABSTRACT-The association between an epiphytic stenolaemate bryozoan, Lichenopora novaezelandiae, its algal host substratum Agarum fimbriatum (Order Laminariales). and an herbivorous snail, Tegirla pulligo (Subclass Prosobranchia), was investigated. Field surveys showed that bryozoan age distributions were spatially correlated with algal age distributions: younger bryozoans were concentrated on the youngest parts of the algae, and vice versa. In laboratory experiments, planktonic larvae of this bryozoan displayed a strong preference for A. fimbriatum over other kelps, and preferred settling on younger regions of A. fimbriatum over older regions. Although 7: pulligo was deterred from feeding on A. fimbriatum as compared to other kelps, the snails nonetheless displayed within-thallus discrimination for tissues of different ages of this low-priority food item. Younger tissues, with or without resident epiphytic bryozoans, were consumed in much lower quantities than older tissues. Adult bryozoans on older A. fimbriatum deterred feeding by the gastropods, revealing an indirect beneficial effect of epiphytic organisms on their host algal substrata. Previous dlet of 7: pulligo maintained in the laboratory failed to influence significantly the snails's consumption of A. fimbriatum.