Tetraploid cells of D.melanogastergrown in culture (line Ca.) characterized by one or more Y chromosomes (of different length), make it possible to describe the behaviour of these heterochromatic elements during prophasic synapsis. In this stage the Y chromosomes (as well as the other heterochromatic portions of the complement) are clearly allocyclic with respect to euchromatin, since they do not show a real synapsis. Nevertheless, a more specific attraction within the Y's exists, since all these elements show a tendency to form a group apart.