The role of herbivores in maintaining the pattern of zonation of macroalgae in the mid-intertidal community of semi-exposed shores was studied during 8 mo. (Jan.-Sept., 1979). Herbivores were totally removed every 15 days from a semi-exposed intertidal rocky wall, while another wall served as a control. Percent cover of algae was measured at 2 mo. intervals. Herbivore removal resulted in a disruption of the pattern of zonation. Iridaea boryana and Gymnogongrus furcellatus expanded the width of their belts into the zone initially occupied only by Ulva rigida and Lithophyllum spp. Herbivory is the basic mechanism which maintains the limits between the zones of these macroalgae in southern Chile.