Vocal patterns from two populations of common dolphin (Delphinus delphis) were compared. Field observations took place in the Channel Islands, CA and La Paz, Mexico. Whistles were recorded with a Sony HTI hydrophone (range=10–32 000 kHz) and schools were observed for group number and distance to the hydrophone by onboard observers. Sounds and spectrograms were analyzed for mean intensities in the (a) 15- to 20-kHz bandwidth, and (b) the ratio of the intensities from the 15- to 20-kHz band compared to the whole spectrum, 0–22 kHz. Significantly different (p<0.001) sound intensity levels were found between the La Paz and Channels Islands populations. When the 15- to 20-kHz band was examined, the values were 104.70±1.73 dB (mean ±S.E.) and 86.80 ±0.90 dB for the La Paz and Channel Islands, respectively. When the 15- to 20-kHz band was looked at as a function of the total energy in the 0 to 22-kHz band, significantly different energy levels were present as well (La Paz=76.2%, Channel Islands=67.9%). Therefore, the two populations were shown to be significantly different (p=0.001). This suggests that acoustic geographic variation is a useful tool for distinguishing populations within a species, and raises the possibility of external influences determining vocal patterns.