ABSTRACT: A recently developed motion detector (acceleration data-logger), based on acceleration measurements, was used to monitor the swimming behavior of two free-swimming captive rainbow trout Oncorhynchus mykiss in an aquaculture net cage. Depth, swimming speeds and two-direction acceleration data were collected continuously for approximately 20 h per fish. To define relationships between swaying acceleration profiles and tail beat activity of rainbow trout, the tail beat activity of trout was monitored using a video camera in a small tank with the simultaneous use of the acceleration data-logger. During steady swimming, there were sharp, distinct peaks of swaying acceleration. When compared with the data from the video camera, the frequency of swaying acceleration was synchronized with the sequences of swimming activity. In addition, the tail beat frequency of the fish could be identified within the cycle of swaying acceleration during steady swimming phases. Mean tail beat frequency was 1.27 ± 0.40 and 1.40 ± 0.5 Hz (mean ± SD). Using the relationship between tail beat frequency and swimming speed, the ‘preferred’ swimming speed of trout was estimated to be between 0.48 and 0.58 body length (BL)/s, and trout rarely swam in excess of 2.0 BL/s. The present study shows that acceleration data-loggers used to record spontaneous measurements of swimming speed and tail beat activity represent a useful and reliable system for accurately estimating both the rate of activities and movements of free-ranging fish.