Habituation of the landing response of Drosophila is described by two behavioural variables. The landing activity and the time till the onset of the response. These data are described in the framework of a leaky integrator model. It is shown that the course of habituation can be simulated by varying only one of the parameters of the model, the output of the detector for landing stimuli. The two other parameters, the leakiness and the degree of biological variance, are kept at fixed values. With the exception of the data for mutant rutabaga, the same values for the constant parameters can be used to describe the data for two wildtype strains and mutants defective in olfactory learning. Duerr and Quinn (1982) have shown that these mutants habituate slower than the corresponding wildtype to chemosensory stimuli. In contrast habituation to visual stimuli of the mutants dunce amnesiac and rutabaga is faster than that of wildtype Canton-S. The theoretical description shows that this can not be attributed to leakier integrators of the mutants, but rather to a faster decrease of the output of the landing detector.