During movement, the body angles of ribbonfish change, including a unique vertical swimming motion that influences their target strength (TS). In this study, we observed body angle changes in ribbonfish using an acceleration data logger and video recordings. Based on these data, variations in echo intensity in response to body angle changes were revealed using radiograph images of the swim-bladder and the respective TS values. The frequency of a body angle >60 degrees was 57.78% peak at 77 degrees. The swimbladders were found to be slender ellipses inclined 3-4 degrees upward from the body axis. The TS peaked (about -30 dB for the average pre-anal length of 223.9 +/- 21.8 mm) when the body angle was horizontal or inclined 2-3 degrees downward. The TS decreased substantially when the body angle was vertically inclined. From about 10 degrees upward inclination to vertical, the TS fluctuated between -50 and -60 dB. In a comparison of the average TS values among body angle categories, the TS values for body angles grouped as 'vertical' (between 60 degrees and 90 degrees), showing static mode activity patterns in video recordings, were low (-55.7 +/- 3.9 dB, Mean +/- SD), whereas those for the horizontal body angle (between 0 degrees and 30 degrees) category, comprising the active mode, were high (-47.0 +/- 8.7 dB) (P-value < 0.01). In the natural habitat, ribbonfish are known to gather at the sea floor during the daytime. In contrast, at night they disperse and move actively while foraging. Consequently, their echo intensities in natural habitats change substantially in response to their diel changes in behavioural ecology. Additional measurements or theoretical models considering the influence of ribbonfish behaviour should be undertaken as a result of this study.