In the British Isles, oligophagous marine herbivores, particularly the ascoglossan (= sacoglossan) sea slug Elysia viridis, associate with the introduced green macroalga Codium fragile ssp. tomentosoides. Slugs prefer to associate with and consume the introduced C. fragile to the native C. tomentosum. Our investigation of adult, larval, and juvenile E. viridis focused on whether this association is attributable (1) to a host switch or (2) to an expansion from native hosts to the introduced C. fragile ssp. tomentosoides. Growth rates and maximum body sizes of E. viridis on introduced hosts were greater than on natives. Although the native Cladophora rupestris induced a high rate of slug metamorphosis, recently metamorphosed juvenile E. viridis (from Codium fragile-feeding parents) were generally not able to feed or grow on the native alga. In contrast, juveniles from Cladophora-feeding parents could eat Cladophora, although their performance was highly variable. Small, postlarval slugs (<800 μm long) could not effectively puncture cell walls and extract algal cytoplasm and chloroplasts from Cladophora; slugs fed far more readily on the thin-walled C. fragile. The new association appeared to be a host switch such that Codium fragile feeders and their offspring had limited capacity to complete their life cycle on the native host Cladophora. Larval metamorphosis in E. viridis was greatest on potential host species, but larvae also responded to nonhost macrophytes and adult conspecifics. The generality of larval host-plant selection and larval metamorphosis enables oligophagous adult consumers to exploit spatially unpredictable novel hosts, to capitalize on newly available host plants, and to exhibit spatial and temporal variation in host-plant associations.