In the yeast Saccharomyces cerevisiae three G1-S cyclins, or CLNs, have been identified that trigger the G1-S transition of the cell cycle. The regulation of the G1-S transition is particularly intriguing in Drosophila both because G1 is added to the cell cycle developmentally and G1-S regulators may drive the polytene cell cycle. To identify potential G1-S regulators from Drosophila, a cDNA expression library was constructed in which Kc cell cDNAs were placed in a high-copy S. cerevisiae vector under the control of the constitutive ADH1 promoter. Following transformation into an S. cerevisiae strain lacking all three CLN gene products, we identified one Drosophila cDNA that complemented the yeast G1 cyclins and restored growth to near wild-type levels. The CLNDm gene is present as a single copy in the Drosophila genome and encodes a 1.2-kb mRNA. DNA sequence analysis reveals that although this gene has cyclin homology, it is a new member of the cyclin gene family. CLNDm mRNA expression correlates with periods of maximal cell division throughout Drosophila development. The transcript is most abundant in early embryos, and it is present in low levels in larvae, pupae, and adults. Drosophila embryos hybridized in situ to this cyclin gene show uniform expression of the message throughout the embryo, with diminishing expression as embryogenesis proceeds.