This paper describes the sonar sounds from four captive Neophocaena, two males and two females, which were tape recorded and processed with regard to their behaviour in the time and in the frequency domain. The echolocation-type pulses produced by the animals fall into two categories, with dominant frequencies of 128 kHz. The time duration of the typical waveforms ranges from 40 to 80 µsec with a number of cycles of 5 to 10. A qualitative inspection of the overall wave shapes shows a remarkable similarity to signal types from Ph. phocoena and C. commersonii. Neophocaena sonar turns out to be another example of an optimal signal in a time-frequency duration with a figure only 15% above the theoretical lower bound of 1. Clear differences in reverberational aspects of different sonar pulses were noted, possibly indicating individual features. No frequency modulation was detected, as the low-valued uncertainty product indicates. There was no indication of any whistle activity pattern during the recording period.