The baker's yeast Saccharomyces cerevisiae can undergo pseudohyphal differentiation upon limited starvation for nitrogen. This differentiation is characterized by a hyperpolarized cell growth that gives rise to elongated cells. These elongated cells can form chains that penetrate an agar surface. The study of the grr1 mutant, affected in the degradation of the G1 cyclins, showed that the stabilization of Cln1 and Cln2 leads to a similar hyperpolarized cell growth. We suggest that G1 cyclin stability is a key element controlling cellular morphogenesis. Examination of G1 cyclin turnover during pseudohyphal growth strongly supports this hypothesis. Saccharomyces cerevisiae is thus an interesting model for studying the interconnections between cell cycle control and cellular differentiation.