The Saccharomyces cerevisiae Snf1 protein kinase has been reported to be required for adaptation to glucose limitation and for growth on non-fermentable carbon sources. Here we present novel findings indicating that Snf1, the key regulator of cellular energy, is also involved in yeast cell cycle control. The lack of Snf1 α-catalytic subunit down-regulates the growth rate and CLB5 expression, delaying Sld2 phosphorylation and G 1/S transition, in cells grown in 2%, but not in 5% glucose. A non-phosphorylatable Snf1 rescues the slow growth phenotype, whereas a wild type or a phosphomimetic mutant is required to rescue growth rate and the G 1/S delay. Using either Snf1 or Swi6 as a bait, a specific interaction of Snf1 with Swi6, the regulatory subunit of MBF, was detected. In conclusion, this report describes a previously unrecognized role for Snf1 in transcriptional modulation of the G 1 to S transition, differing from the reported AMPK role in controlling the G 1/S transition in multicellular eukaryotes.