Abstract When attacked by a predator, sea hares may release two chemically defensive secretions, ink and opaline. These secretions are mixed in the mantle cavity and then released towards the attacker. In this study of the sea hare Aplysia californica, we present data showing that both secretions and hydrogen peroxide (H 2O 2), a product of the reaction between escapin, an L-amino acid oxidase in ink, and lysine in opaline, are able to elicit aversive behaviors from spiny lobsters, Panulirus interruptus. The behaviors include mouthpart rubbing and tail flipping (for opaline and H 2O 2), deterring feeding (for ink and H 2O 2), or completely inhibiting feeding (for opaline). The aversive behavior is not elicited through long distance chemoreceptors located in the antennules, as shown by ablation experiments, nor is it due to nonspecific stimulation, as shown by the failure of KCl and highly concentrated positive stimuli to elicit it. A concentration-response curve shows that the ED 50 for H 2O 2 is 13.7 mM, an amount well below what is attainable in situ. The effect on feeding is demonstrated by the fact that lobsters do not eat shrimp laced with full strength opaline and spend a significantly greater amount of time handling shrimp laced with ink, H 2O 2, or opaline than handling control shrimp. Taken together, these data suggest that opaline, ink, and their mixture contain compounds, one of which is H 2O 2, that can alter a predator's behavior to allow the sea hare to escape attack.