Elysia viridis (Montagu) occurs naturally in populations feeding on the fleshy, siphonalean green alga Codium or on the coarse, filamentous green alga Chaetomorpha. Only about half of the E. viridis transferred from Codium to Chaetomorpha in the laboratory were able to learn to feed on this alga. Handling time decreased significantly during the learning-period. All E. viridis transferred from Chaetomorpha to Codium in the laboratory were able to learn to feed on the latter alga. Handling time also decreased with experience in these animals. Ingestive conditioning occurred in animals transferred from Chaetomorpha to Codium. These animals had to go through a learning-period again when they were offered Chaetomorpha after 6-9 weeks with Codium Ingestive conditioning was also evident from preference experiments; animals preferred the food they had been kept on prior to the experiments. These findings are discussed in relation to optimal diet theory.