The chromosomal location of the two genes that encode the cytochrome P450 enzymes, P450SCC (cholesterol side-chain cleavage) and P450arom (aromatase), was identified in the mouse. Genomic DNA from several progenitor strains of recombinant inbred (RI) strains of mice was tested with various restriction endonucleases for restriction fragment length variations. Variation in Bam HI fragment length was detected between A/J and C57BL/6J. Genomic DNA from 43 RI strains derived from A/J and C57BL/6J was analyzed in a similar manner. Complete concordance of the strain distribution pattern for P450SCC and that of P450arom was observed for 43 RI strains. The lack of recombination indicates that the structural genes encoding P450SCC and P450arom are closely linked. The strain distribution patterns of the P450SCC and P450arom genes were compared with other markers previously mapped in these RI lines. The results demonstrate that both P450SCC and P450arom are found on mouse chromosome 9. Of the other loci on mouse chromosome 9, P450SCC and P450arom are most closely linked to the gene encoding P1450. Among 31 RI strains for which the three loci were analyzed, only one example of discordance was found. Human P450SCC, P450arom and P1450 have been mapped to human chromosome 15. However, the distance between the human P450SCC gene and other loci has not been determined. The information presented in this report, along with other studies, indicate conservation between homologous human and mouse chromosomal regions and suggest that human P450SCC will be found to be closely linked with human P450arom.