Finless porpoises (Neophocaena phocaenoides) are distributed in Asian waters. The narrow-band and high-frequency pulse sonar signals produced by this species are distinctive from background noises. Underwater sound monitoring by a hydrophone (BK8103) along board sides of a research vessel concurrent with visual observation were conducted in the Yangtze River from Wuhan to Poyang Lake in 1998. The peak-to-peak detection threshold level was 133 dB re: 1 μPa, which could be roughly converted to the theoretical detection range of 316 m, hypothesizing the porpoise directing to the hydrophone and 168-dB source level of the sonar signal under a shallow water sound propagation (combination of spherical and cylindrical spreading). In a total of 1064-km cruise, 717 finless porpoises were observed. The acoustical monitoring system could detect the sonar signals from finless porpoises, found within 200 m from the research vessel. The sonar signals could also be detected at night and under windy weather conditions. Basically, the acoustical observation system was operated automatically and free from the individual difference of observers. The high-frequency acoustical monitoring seems to be an effective method for the survey of small cetaceans which produce sonar signals.