Foraging behaviour of Porcellio scaber was observed in laboratory arenas in which the spatial distribution of patches of high quality food (powdered dicotyledonous leaf litter) was varied within a matrix of lower quality food (powdered grass leaf litter). The hypotheses that feeding behaviour of isopods would vary with the degree of clumping of high quality food patches and with the density of conspecifics, were tested. In more clumped treatments, animals spend less time on high quality food and more on a low quality one. At higher densities more time was spent searching. This effect was more pronounced in clumped treatments, but negligible in homogeneous ones. The effects of variation in the spatial heterogeneity of high quality foods on the trade-off between searching costs and intake-rate benefits for saprophages are discussed in the context of predictions from optimal foraging theory for scenarios in which intake-rate maximisation is constrained by nutrient limitation.