The present differential Pavlovian conditioning experiments on the sea slug Pleurobranchaea californica extend conditioning described in a preceding paper and provide the conditioning foundation for studies reported in another accompanying paper comparing learned behavior in whole animals with the behavior and motor patterns of electrophysiological preparations. All animals received two appetitive-conditioned stimuli (CSs), one derived from beer (Sbr) and the other derived from squid muscle (Ssq), in different temporal relationships to an electric shock unconditioned stimulus (UCS). Two groups of animals were run concurrently. One group (n = 19) received Sbr as the CS+ in close temporal pairing with the UCS, and Ssq as the CS- explicitly unpaired with the UCS (Sbr +/Ssq-). The second group (n = 20) received the opposite contingencies (Sbr-/Ssq+). All animals received only one day of conditioning involving 5 trials with an intertrial interval of 2 h. There were two replicate experiments, each involving about half of the total n, and each yielding similar results as the sum we report here. Before conditioning, animals exhibited feeding behavior (extension of the proboscis and bite-strike responses) to both stimuli at similar low thresholds. Conditioning produced long-term behavioral changes in all animals throughout the 4.5-day postconditioning observation period. However, only the Sbr+/Ssq- animals consistently exhibited the appropriate differentially conditioned food-aversion behavior which consisted of strong withdrawal and high-threshold feeding responses to Sbr, and low-threshold feeding responses to Ssq. We discuss the possibility that such differences between Sbr+/Ssq- and Sbr-/Ssq+ conditioning may arise either from inherent differences in the responses of the animals to Sbr and Ssq, or, as seems more likely to us, from training and testing effects produced by differences in the compositions of the two stimuli.