Host plant choice by herbivores may be constrained by the spatial arrangement of plants with selection of poor quality hosts being more likely if preferred hosts are locally rare. I tested whether the presence of a herbivorous marine amphipod, Peramphithoe parmerong, on a poor quality host could be explained by the relative abundance and spatial arrangement of alternative brown algal hosts. Amphipods strongly preferred the high quality Sargassum linearifolium over the poor quality Padina crassa in laboratory assays with a higher tendency to select Padina crassa when it was relatively more abundant. In the field, however, adults colonised both hosts to equal densities. Rates of colonisation were independent of host quality but strongly affected by the identity of neighbouring hosts with those algae in close proximity to S. linearifolium receiving far greater densities of amphipods than those surrounded by Padina crassa. There was no evidence that the presence of adult Peramphithoe parmerong on Padina crassa was due to local scarcity of S. linearifolium. In contrast, the distribution and behaviour of juveniles were predictable from differences in food quality, with most inhabiting the high quality S. linearifolium and few emigrating from this host. The success of a simple adaptive model based on food quality to predict juvenile, but not adult, behaviour indicates that the relative importance of food quality varies with age, and that other factors are important determinants of adult movements among hosts.