The assemblage of marine invertebrates associated with floating macroalgal rafts may potentially be structured by ecological interactions and the characteristics of the rafts themselves. Alternatively, the assemblages might reflect a random process of colonization and loss from fragments, such that there is no systematic variation in assemblage structure with raft type. Analyses of 51 rafts collected from Irish coastal waters found no evidence for consistent patterns in the occurrence of species with raft size or algal composition. The observed patterns could not be distinguished from the predictions of a model with random allocation of individuals to rafts. Invertebrate species richness increased with raft size, but a multivariate test showed that this pattern was not associated with consistent changes in the identities of species found. Macroinvertebrate species assemblages were not significantly structured by the different species of algae in the raft. In contrast, the environmental variables at the point of raft collection, along with location data, were associated with variation in the species composition of rafts. The absence of any systematic structure related to raft size or composition in the sampled rafts suggests that, for the region studied, processes of raft break-up and coalescence obscure any potential effects of ecological interactions on individual rafts. The location-related variation in rafting invertebrate populations may reflect spatial variation in source populations or environmental factors that alter species compositions independently of raft type.