This study examines the mechanisms of aposematism (unprofitability of prey combined with a conspicuous signal) in the aeolidioidean Cratena peregrina (Gmelin, 1791). We investigated if marine fish avoid attacking aeolidioidean nudibranchs, which are an unprofitable group of prey for most predators. We analysed the interaction between aeolids and predatory fish in laboratory and field assays, with both live aeolids and artificial models. In the first experiment, we offered normal and blue-dyed C. peregrina to fish in the field. The number of attacks by fish was independent of the density of the prey, albeit the normal aeolids were attacked less frequently than the blue ones. The fact that all normal C. peregrina survived, whereas 12–20% of the blue aeolids died after fish attacks, suggests that aposematic coloration provides a selective advantage against fish predators. Field and laboratory assays with artificial aeolids were employed to test the effects of different factors (number of cerata, colour pattern, nematocysts), or a combination of factors, on fish learning. Fish learned to avoid unpalatable models with the colour pattern of C. peregrina. After three to four training sessions with unprofitable models, fish avoided profitable models with the same colour pattern. Our results suggest that the colour pattern of C. peregrina combined with the presence of dorsal appendages and nematocysts make fish avoid aeolids.