This study investigated the physical and physiological response of individual setae on the antennule of Gaussia princeps. We found significant differences in the physical and physiological responses of the setae to various intensities of water flow. No physiological evidence was found to suggest that individual setae are dually innervated; however, directional bias in both the displace- ment and subsequent physiological responses was evident. Although more easily displaced by fluid flow, the shortest hairs were physiologically less sensitive to angular deflection than were the longer setae, so that slow flows produced a greater neural response in the long seta. The combination of high resistance to movement and acute physiological sensitivity allows the long seta to respond to biolog- ically driven, low-intensity flows while filtering out high-frequency background noise. This suggests that the most prominent, long, distal setae function as low-flow detectors whereas the short hairs respond to more rapid fluid motion. Each seta responds to only a portion of the overall range of water velocity in the copepod's habitat. Thus, the entire sensory appendage, which consists of an ensemble of setae of different morphologies and lengths, may function as a unit to code the intensity and direc- tionality of complex fluid disturbances.