1. Alignments of the available cytochrome P-450 reductase amino acid sequences, and comparison with the crystal structure of ferredoxin-NADP reductase, indicate that two highly conserved regions are of functional importance. 2. Degenerate oligonucleotide primers, based on these sequences, were used in the polymerase chain reaction to amplify a 309 bp fragment of the cytochrome P-450 reductase gene from Schizosaccharomyces pombe for use as an homologous probe. 3. A 2.6 kb cDNA was cloned from a lambda library, and sequencing revealed an open-reading frame of 2034 bp encoding a protein of M(r) 76774. This protein shares 38-41% identity with other eukaryotic cytochrome P-450 reductases, and 30% identity with that of Bacillus megaterium. 4. Comparison of the N-terminal FMN-binding domain with flavodoxin, and the C-terminal FAD- and NADP-binding domain with ferredoxin-NADP reductase, indicates the presence of several functionally conserved regions. 5. The Sc. pombe cytochrome P-450 reductase gene was shown to contain no introns.