Experiments with crucians (Carassius carassius L.) and goldfish (Carassius auratus L.) have shown that a consolidated memory trace of a simple visual pattern became disturbed by later seeing series of similar patterns (unrewarded), that is to say by latent learning. In a test in which the experimental group (es) had to choose between the originally learnt and the similar pattern, it did not prefer the former, whereas the control group (cs), which had only seen white walls of the aquarium, still preferred the originally learnt pattern. The same happened when es learnt a similar pattern by reward after the memory of the original pattern had been consolidated. In another set of experiments the negative influence of post-exposure could be demonstrated in facilitation of later learning the postexposed pattern as positive versus the originally learnt pattern, now negative. When es had seen a dissimilar wall-paper pattern then the memory of the primarily learnt pattern was not disturbed. A tentative explanation is based on the notion that two similar engrams have part of the neuronal network in common.