The pre-imaginal development ofDrosophilamushroom bodies is under the influence of an unknown variable which causes populations of wild-type flies at eclosion to differ in the average number of Kenyon cell fibers. During the first week of adult life the number adjusts to an intermediate level which depends upon the experience of the flies. Under olfactory deprivation or social isolation it reaches a lower level than under favorable rearing conditions (/.Neurogenet., 1 (1984) 113–126). The biochemical learning mutantsdunceandrutabagashow no experience-dependent modulation of fiber number (Fig. 2). In both strains the mushroom bodies of young adults seem to develop abnormally: induncea loss of about 600 fibers is observed, inrutabagafiber number is low at eclosion and does not increase (Fig. la). The following model for long-term memory is proposed: in mushroom bodies outgrowth and decay of Kenyon cell fibers occur simultaneously. The fibers randomly form transient synapses onto extrinsic output neurons of the mushroom bodies and receive synapses from modulating neurons. Experience consolidates certain synapses, thus prolonging survival of the respective Kenyon cell fibers and increasing the steady state level of fiber number (Fig. 3).