The expression of cyclin D1 in mid-G1 phase is associated with sustained ERK activity, and we show here that Rho is required for the sustained ERK signal. However, we also report that Rho inhibits an alternative Rac/Cdc42-dependent pathway, which results in a strikingly early G1-phase expression of cyclin D1. Thus, cyclin D1 is induced in mid-G1 phase because a Rho switch couples its expression to sustained ERK activity rather than Rac and Cdc42. Our results show that Rho is crucial for maintaining the correct timing of cyclin D1 expression in G1 phase and describe a new role for cytoskeletal integrity in the regulation of cell cycle progression.