The unusual two-track type of division which is widespread in most fungi other than Phycomycetes is described. Three models which attempt to account for this type of division are discussed in relation to (i) genetical observations on somatic segregation and (ii) chromosome distribution studies which predict that chromosomes of similar genetic 'age' move as a set. The feasibility of two of the models which postulate that the chromosomes are linked in chains is discussed in the light of our knowledge of mitotic crossing-over and haploidization. The third model (Robinow and Caten 1969) is more orthodox and differs from classical mitosis only in the presence of a narrow intranuclear spindle around which the chromosomes attach randomly and disjoin asynchronously. Future experimental approaches to distinguish between these models and to define the mechanism of division more completely are suggested.