The exposure to discharging nematocysts of the eolid nudibranch Cratena peregrina feeding on polyps of the hydroid Eudendrium racemosum was examined. The smaller size-class of microbasic eurytele mastigophor nematocysts is located on the tentacles of the hydroid polyps, while the larger size-class of holotrichous isorhiza nematocysts is located on long cnidophores and basal rings of some polyps. Discharging nematocysts of the tentacles do not appear to have damaging effects on the dorsal appendages of the snail, the cerata. In contrast, contact of a cnidophore with a ceras elicits massive discharge of nematocysts, which results in adherence, membranolysis and local destruction of the epidermis within seconds. The differential locations and effects indicate that the small tentacle nematocysts serve for capturing prey, while the large cnidophore nematocysts serve for the hydroid's defence. Both small and large, discharged and intact nematocysts were found in the pharynx and stomach of the snail after feeding. Thus, the alimentary tract appears to be efficiently protected against discharging nematocysts. Small and large intact nematocysts, which were capable of discharge, were also found in the cnidosacs.