Yarrowia lipolytica has two mating types designated A and B. Mating type A cells produce mating specific pheromone (A factor). Crude fractions of the A factor was partially purified and concentrated (1.000 fold) by Amberlite CG 50. This A factor acts only on cells of opposite mating type inducning a cascade of metabolic and morphological changes. Ths A factor specially inhibits the cell division of mating type B. The cell were arrested in the G1 phase (no budding, characterized by light microscopy). The arrest is reversible and the lenght of the arrest period is dependent on the concentration of A factor added. The A factor is one element of a relatory system whose functions assure the synchronization of haploid cells for conjugation. In an asynchronous population only a small part of cells are in this specific stage. When A factor is added to conjugation medium (YM), the time for conjugation was shorter and the conjugation rate was increased.