ABSTRACT This paper proposes a general model for interference in genetic crossing over. The model assumes serial Occurrence of chiasmata, visualized as a renewal process along the paired (or pairing) chromosomes. This process is described as an underlying Poisson process in which the Ist, n + 1th, 2n + 1th, etc., events are to be interpreted as realized chiasmata. Chromatid interference is described in terms of the probabilities that two successive chiasmata involve two, three or four different chromatids. Several characteristics of this model, e.g., the cytological and genetic mapping function and the density of chiasmata along the chromosomes, are discussed. Some aspects of other interference models are briefly discussed.