In most sensory modalities, neuronal inputs are bilaterally processed in a higher center. In some animal species, however, functional lateralization is sometimes observed in the sensory processing at the higher level. For the terrestrial slug Limax, olfaction is the most important sensory modality and this slug can acquire odor-aversion memories. Previously, it has been demonstrated in bilateral PC ablation experiments that the procerebrum (PC) is necessary for odor-aversion memory, and that the PC is the memory storage site. On the other hand, it has been hypothesized that only the unilateral PC is used for odor-aversion learning. Here we demonstrated that the number of the slugs with intact memory performance was reduced by approximately 50% when the PC was surgically ablated only unilaterally before or after conditioning. There was no difference in the memory performance of the right vs. the left PC-ablated slugs. However, memory deficit from unilateral PC ablation was not observed when the ipsilateral tentacles were also amputated at the same time. We also showed that there was no lateral memory transfer from one PC to the other, after up to 7 days post-conditioning. Our results demonstrated clearly that either the left or right PC is randomly used for olfactory learning, and that the side of use is determined at the level of the olfactory ascending pathway to the PC.