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In recent 10 years,a miniature stereo acoustic data logger (A-tag,W20-AS,Little Leonardo,Japan) has been developed and used by us to observe the bio-sonar behavior of free-ranging Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis).The A-tag is small enough to be attached on the one of the smallest odontocetes by using suction cups.The A-tag with two external hydrophones can record the sonar pulse intensity,inter-click-intervals,and sound source direction measured by time arrival difference of sounds at the two hydrophones.Major outcomes by A-tag were the in situ sonar beam pattern,attention and approach phase of biosonar behavior,scanning sonar of rolling animals,and social contacts among free-ranging Yangtze finless porpoises.In addition,frequent ultrasonic sound production seemed to be a reason of high detection performance by passive acoustic monitoring of this species.This biosonar monitoring method using miniature data logger system developed in a freshwater system in China provided new insights of underwater sensing behaviour of other small odontocetes producing high-frequency echolocation clicks,such as harbour porpoises and white-beaked dolphins.