Sotalia fluviatilis was the only cetacean species observed in all 16 boat surveys (82 h) conducted in the Sepetiba Bay (22° 58′S, 44° 02′W), between December 1993 and August 1995. Previously, S. fluviatilis whistles were bioacoustically poorly characterized. The objective of this research was a quali–quantitative characterization of tucuxi’s acoustic emissions of descent, up–down, and down–up frequency. The sound analysis was based on sonograms from 30 min of a subaquatic recording obtained on 5 January, 1995. The qualitative analysis resulted in 390 whistles classified in 3 subclasses (descent, up–down, and down–up frequency), 21 types, and 46 subtypes. The quantitative analysis was based on: number of harmonics; number of inflexions; beginning, final, maximum, and minimum frequencies; duration; and frequency modulation. Minimum, maximum, and average values, standard deviation, and coefficient of variation were calculated for each type and subtype of whistle. The S. fluviatilis whistles are extremely diverse (high coefficient of variation values for each subclass, type, and subtype) and frequent (38.5 whistles/min), seeming to have an important role in the social relations among these animals. Such whistle diversity and quantities could be explained by the composition of the dolphin group, i.e., it was exclusively formed by young and subadult animals.