The cdr1 gene in Schizosaccharomyces pombe was identified as a mutation affecting the nutritional responsiveness of the mitotic size control. cdr1 alleles have been further analyzed for genetic interactions with elements of the mitotic control pathway and cloned by plasmid rescue of a conditional lethal cdr1-76 cdc25-22 double mutant. These analyses show that the cdr1 gene is allelic to nim1, a gene identified as a high copy number plasmid suppressor of the mitotic control gene, cdc25. The gene structure for cdr1 differs from the described nim1 gene in the carboxyl-terminal portion of the gene. The published nim1 sequence encoded a product of predicted Mr 45,000, and included 356 amino acids from the amino-terminal region of the gene and 14 amino acids from a noncontiguous carboxyl-terminal fragment. The cdr1 sequence includes an additional 237 amino acids of the contiguous fragment and encodes a product of predicted Mr 67,000. The sequence shows a high level of identity with protein kinases over the amino-terminal catalytic domain, and limited identity with yeast protein kinases SNF1, KIN2 and KIN1 over part of the carboxyl-terminal domain. The effect of overexpression of the full length gene has been examined in various genetic backgrounds. These data show that the full length gene product is required to give a normal cell cycle response to nitrogen starvation. A detailed examination of the genetic interaction of cdr1 mutants with various mutants of mitotic control genes (cdc2, cdc25, wee1, cdc13) demonstrated strong interactions with cdc25, some cdc2 alleles, and with cdc13-117. Overall, the results are interpretable within the framework of the existing model of cdr1/nim1 action in mitotic control, i.e., cdr1 functions upstream of wee1 to relieve mitotic inhibition.