Yeast cells overexpressing the Ser/Thr protein phosphatase Ppz1 display a slow-growth phenotype. These cells recover slowly from α-factor or nutrient depletion-induced G1arrest, showing a considerable delay in bud emergence as well as in the expression of the G1cyclins Cln2 and Clb5. Therefore, an excess of the Ppz1 phosphatase interferes with the normal transition from G1to S phase. The growth defect is rescued by overexpression of the HAL3/SIS2 gene, encoding a negative regulator of Ppz1. High-copy-number expression of HAL3/SIS2 has been reported to improve cell growth and to increase expression of G1cyclins in sit4 phosphatase mutants. We show here that the described effects of HAL3/SIS2 on sit4 mutants are fully mediated by the Ppz1 phosphatase. The growth defect caused by overexpression of PPZ1 is intensified in strains with low G1cyclin levels (such as bck2Δ or cln3Δ mutants), whereas mutation of PPZ1 rescues the synthetic lethal phenotype of sit4 cln3 mutants. These results reveal a role for Ppz1 as a regulatory component of the yeast cell cycle, reinforce the notion that Hal3/Sis2 serves as a negative modulator of the biological functions of Ppz1, and indicate that the Sit4 and Ppz1 Ser/Thr phosphatases play opposite roles in control of the G1/S transition.