Bottlenose dolphin whistle structure was compared among populations, by spectral and statistical analysis. Differences in whistle structure were greater between far separated than closer areas. Animals from nearby areas may influence each others' whistles by mimicry, but may also have similar whistles due to periodic change of individuals across areas. Whistle differences among groups in adjacent areas were small. Dolphins in nonadjacent areas presumably developed unique acoustic characteristics due to geographic isolation. The characteristics of whistle structures could potentially be used to distinguish among populations. Frequencies of the major energy of dolphin whistles were higher than the major frequencies of background noise, and a specific acoustical niche relative to environment may be hypothesized.