SUMMARYNear-field disturbances in the water column are known to trigger reflex antennular flicking in the crayfish Procambarus clarkii. We have identified the hydrodynamic sensors on the lateral antennular flagellum that constitute an afferent limb of this reflex and have measured the relative directionally dependent thresholds of the sensory neurons associated with these structures to hydrodynamic stimulation. Twenty-five individual standing feathered sensilla, comprising a sparse, linearly arrayed population of near-field sensors along the lateral and medial antennular flagella, were exposed to standardized pulsatile stimuli at 20deg intervals along a 320deg circular track. The results indicate that the sensilla are most sensitive to such stimulation in the plane of the flagellar axis. Identification and mechanical stimulation of single feathered sensilla in some preparations consistently evoked a flick reflex at maximal response latency, indicating that these sensors constitute at least one afferent limb for the reflex behavior. Experiments in which response latencies were measured following mechanical stimulation of truncated flagella, and were compared with the latencies in respective intact flagella, suggest that summation of inputs from the feathered sensillar pathways generates reflex flicking at minimal latencies. We discuss the possible central mechanisms that may underlie detection of critically important signals from this population of highly sensitive, inherently noisy sensors.