Wee1 is one of the genes which regulates the cell cycle. cDNA encoding a Wee1-homolog has been isolated from Japanese silver crucian carp (ginbuna) by using polymerase chain reaction (PCR) techniques. The cDNA obtained from the ovary tissue encoded a protein of 526 amino acids and the deduced amino acid sequence showed high homology (69.9%) with Xenopus laevis Wee1 in the protein kinase domain. A phylogenetic analysis among Wee1 amino acid sequences from various organisms showed that the crucian carp Wee1 was closely related to Xenopus Wee1 and human Wee1B. The vertebrate Wee1s were classified into two types: the meiotic type and the mitotic type. The crucian carp Wee1 described here belonged to the meiotic type. Furthermore, a long-PCR technique allowed us to isolate the wee1 gene of almost full length. The wee1 gene was about 6.3 kbp in length and contained 12 exons, with the open reading frame starting at the second exon.