We have cloned a novel fission yeast gene, spd1, which causes G1 arrest when overexpressed. Deleting the gene results in cells being accelerated through G1 into S phase in certain circumstances when the G1-->S phase control is compromised. We have found that the encoded 14 kDa protein is cell cycle regulated, declining in level during S phase, and that p14spd1 physically associates with p34cdc2 in vivo when overexpressed, suggesting that p14spd1 may regulate S phase progression via an interaction with p34cdc2. We conclude that p14spd1 is a negative regulator of S phase, and that it may be part of the control ensuring an orderly onset of S phase or part of a G1-->S phase checkpoint control.