The occurrence of small specialist herbivores in marine rocky intertidal communities has been largely ignored. From 1984 to 1989, the phenology and demography of the common oligophagous ascoglossan opisthobranch Placida dendritica A.& H. was documented at several sites along the central coast of Oregon, USA. P. dendritica was found on the low intertidal green algae Codium setchellii Gardn., C. fragile (Sur.) Har. and Bryopsis corticulans Setch. during spring and summer. Ascoglossan abundance on Codium spp. varied spatially and temporally. Peak density usually occurred in May or June. Maximum ascoglossan live mass was 4 to 6 mg on Codium spp. and ∼ 100 mg on B. corticulans. Larval recruitment of P. dendritica to algal hosts was continuous during spring and summer. Rapid recruitment (200 to 400 sea slugs thallus-1mo-1) of newly metamorphosed ascoglossans to C. setchellii in wave-protected areas suggests that high densities of competent larvae occurred in nearshore waters and/or that larvae were efficient at locating algal hosts. Ascoglossan growth was rapid, with less than one month elapsing from larval settlement to adult sexual maturity. Therefore, populations of P. dendritica were composed of many overlapping generations. Movement of adult ascoglossans among thalli of C. setchellii was rapid. Continuous larval settlement and adult movement enabled P. dendritica not only to locate and colonize its algal hosts quickly but also to maintain a persistent association with algal hosts during spring and summer.