The responses of the long, spiny, antennal flagella of the spiny lobster Panulirus argus to hydrodynamic and tactile stimuli were investigated. Experiments were performed in the dark and included videographic laboratory studies of small tethered lobsters (<20 mm carapace length) and nighttime field observations of larger, subadult, foraging animals. The antennae are held laterally in both tethered and free-ranging animals. Water jets trigger bilateral antennal responses in which both flagella are swept forward for rostrally directed stimuli, backward for caudal stimuli, and in an intermediate backward direction when stimulated laterally. Mean response angles are greater for caudal stimuli (17°-48°) than for rostral stimuli (10°-16°), and lobsters exhibit lateralized sensitivity when jets are directed from the caudal sector, as indicated by larger ipsilateral responses--up to twice the amplitude of contralateral responses in field experiments. Untethered lobsters frequently turn the body in the direction of the water jet and tailflip away or tailflip without first turning. Tactile stimuli to the lateral edges of the antenna, carapace, walking leg, abdomen, and tailfan also trigger primarily backward sweeps of the antennae. Only the antennule and medial antennal receptive fields yield forward movements, and these elicit smaller responses (mean response ≤ 5°) than in the backward direction (mean responses up to 15°). Threshold tactile stimuli trigger exclusively ipsilateral responses; thus, lateralization is absolute. These results demonstrate that spiny lobsters accurately localize mechanosensory stimuli and direct their antennal flagella in the perceived direction, a response consistent with a defensive function of the antennae in these nonchelate decapods. Overall sensitivity is greatest for hydrodynamic stimuli, a result interpreted as being important for the detection of and defense against large predatory fish whose nearby movements would generate broad, directional, water-current pulses.