We have cloned and sequenced a DNA fragment (approximately 1 kb) containing a complete open reading frame from a cDNA library of Leishmania donovani promastigotes. The alignment of the derived polypeptide sequence and the modeling studies revealed that the protein is highly homologous to the mammalian cyclins having conserved cyclin box and substrate-docking motif. Northern blot analysis of the RNA isolated from synchronized L. donovani promastigotes showed periodic expression of the message with maximum abundance at S-phase suggesting its involvement in the events related to the regulation of DNA replication. The results confirm that we have isolated a cyclin molecule from L. donovani (LdCyc1) which may play an important role in the regulation of the parasite cell cycle.