This study was planned to determine the neuromuscular basis of the very rapid strike movement, lasting only 5 -1 0 ms, used by mantis shrimps to capture prey or defend themselves. The raptorial second thoracic appendages which are responsible for the strike action are unfurled by four muscles i n the merus, two extensors and two flexors. All were found to be specialized for slow not fast contraction. The largest extensor muscle has a twitch time of 400 ms, reaches peak tetanic tension in 700 ms and takes one second to relax completely from this. It receives two "slow" and one "fast" motor axons distributed uniformly throughout the muscle. An overshooting spike occurs in each muscle fiber from the first activation of a train in the fast axon, but many seconds must elapse before the first muscle fiber will again respond with a spike. The smaller, medial extensor receives one fast axon which may give rise to overshooting spikes and a n inhibitor whose effects appear to be minor; its muscle fibers are only weakly electrically excitable. The contraction and relaxation rates are slow, as are those of the medial flexor which receives two slow axons. The lateral flexor is divisible into two parts; a proximal region comprised of electrically passive fibers innervated solely by a slow axon and a distal region of spiking fibers innervated by a fast axon. The rapid strike action is explained not i n terms of the rapid contraction of these muscles but i n their operation of a mechanical device in previously.