AbstractTheories suggest that food webs might consist of groups of species forming ‘blocks’, ‘compartments’ or ‘guilds’. We consider ecological networks – subsets of complete food webs – involving species at adjacent trophic levels. Reciprocal specializations occur when (say) a pollinator (or group of pollinators) specializes on a particular flower species (or group of such species) and vice versa. Such specializations tend to group species into guilds. We characterize the level of reciprocal specialization for both antagonistic interactions – particularly parasitoids and their hosts – and mutualistic ones – such as insects and the flowers that they pollinate. We also examine whether trophic patterns might be ‘palimpsests’– that is, there might be reciprocal specialization within taxonomically related species within a network, but these might be obscured when these relationships are combined. Reciprocal specializations are rare in all these systems when tested against the most conservative null model.