Two causes of forgetting have been promulgated: memory trace decay and retroactive interference. The authors show that forgetting is an active process requiring both new learning and memory. In the present Lymnaea model system, prevention of new learning of a conflicting association, inhibition of memory consolidation, or Right Pedal Dorsal 1 soma ablation, which blocks LTM formation, are all potent means to prevent forgetting. Thus procedures that alter the ability to learn or form memory of a new conflicting aerial respiratory association prevent forgetting of a learned associative behavior. These results are the 1st demonstration in any model system that forgetting requires the soma of a single neuron.