The behavior of normal Drosophila and of X-linked olfactory conditioning mutants was analyzed by using an olfactory memory choice task. Memory in normal flies was optimal at 5-12 min after training and then decayed slowly. Memory in the mutants was 40-80% of normal when tested 30 sec after training but it decayed rapidly and was very low after 7 min. Successive conditioning experiments indicated that some types of mutants can behave as though they store information for a longer period than that revealed by straightforward memory tests, but this information reveals itself only by interaction with newly acquired information in a conflict situation. It thus seems that the mutations can either prevent the formation of normal memory or prevent memory from evoking a normal behavioral response.