ABSTRACT: We monitored the population size-structure and demography of the intertidal kelp Hedophyllum sessile (C. Agardh) Setchell in areas of manipulated chiton Katharina tunicata (Wood) density to examine size-related 'escapes' and other mechanismswhich may underlie the effects of this herbivore on H. sessile. This research was done on the west coast of Vancouver Island, British Columbia, Canada. We find that K. tunicata is associated with decreased juvenile H. sessile (<1 cmholdfast diameter) survivorship. While young adults of H. sessile (1-4 cm holdfast diameter) were found to survive significantly longer than juveniles between areas of low, control and high chiton density, adult (>4 cm holdfast diameter)survivorship did not differ among the density treatments. We argue that the mechanism underlying the effect of K. tunicata on young adult H. sessile survivorship is an indirect inter-action wherein holdfast integrity is degraded, resultingin an increased susceptibility to wave-induced mortality. These findings provide a quantification of the mechanism of macrophyte size-escapes, may explain the geographically variable effects of K. tunicata and hold implications for studies whichaim to assess the effects of wave exposure on macrophyte survivorship.