Today, sounds produced by cetaceans are used for acoustic detection of individuals and groups in the wild. However, the detection probability ascertained by a concomitant visual survey has not been demonstrated extensively. We studied the finless porpoises Neophocaena phocaenoides in the Yangtze River from Wuhan to Poyang Lake in 1998 in China, using underwater sound monitoring with hydrophones (B&K 8103) placed along the sides of a research vessel, concurrent with visual observations. The peak to peak detection threshold was set at 133 dB re 1 µPa. With this threshold level, porpoises could be detected reliably within 300 m of the hydrophone. In a total of 774 km cruise, 588 finless porpoises were sighted by visual observation and 44,864 ultrasonic pulses were recorded by the acoustical observation system. The acoustic monitoring system could detect the presence of the finless porpoises 82% of the time. False alarms in the system occurred with a frequency of 0.9%. The performance of the acoustic detection system depends highly on the sound production rate and the directionality of the beam pattern. Echolocation click events of two finless porpoises were recorded with an acoustic data logger in an oxbow of the Yangtze River. They produced 3 to 4 click trains in a minute. A behavioural data logger attached to the identical animal provided the dive depth and the body angle simultaneously. Comparing the values of dive depth, the body angle and the level of water surface reflection, the 120 degrees off-axis sonar signals were found to have 160 dB peak-to-peak sound pressure level at one meter from the animal. The finless porpoises produced sonar signals frequently and the off-axis signal had sufficient level to be detected. High frequency acoustical observation is suggested as an effective method for field surveys of small cetaceans, which produce high frequency sonar signals.