The karyotype of a blue whale was studied in a fibroblast culture established from a skin biopsy obtained from a free swimming animal. The chromosome number was 2n=44. Chromosomal analysis showed that the biopsy originated from a female specimen. The karyotype conformed with the general 2n=44 cetacean karyotypes. The C-band pattern had the characteristics observed in other cetacean karyotypes, i.e., interstitial and terminal C-bands being more conspicuous than centromeric C-bands.