Large marine herbivores, such as sea urchins, consume huge amounts of benthic primary production and can control local stocks of marine macrophytes. By contrast, small mesograzers, such as peracarid crustaceans, are abundant on seaweeds but have only low per capita feeding rates. Accordingly, it is difficult to predict their impact on marine macrophyte biomass. We examined the damage induced by the feeding of the herbivorous amphipods Sunamphitoe lessoniophila and Bircenna sp. on the large kelp species Lessonia berteroana in northern-central Chile, southeast Pacific. The amphipods construct burrows in the stipes of subtidal individuals of the kelp inducing a characteristic sequence of progressive tissue degeneration in infested stipes. The composition of the amphipod assemblages inside the burrows varied between the different stages of infestation. Aggregations of amphipods within the burrows and the specific microhabitat preference of the amphipods result in localized feeding, inducing stipe breakage and loss of considerable amounts of algal biomass. The estimated loss of biomass of single stipes ranged from 1 to 77%. For the entire local kelp population, the estimated loss of biomass induced by the herbivorous amphipods amounted to 24–44%. Our results demonstrate that small herbivorous crustaceans can cause huge damage to large kelp species when their feeding activity is focussed on structurally valuable algal tissue.