Amplexus and mating behavior in the horseshoe crab, Tachypleus tridentatus, were studied at imari Bay and Kitsuki Bay, Kyushu, Japan, during Summer, 1994. The pairwise size distribution of mated pairs (n =28) showed a lack of size-assortative mating. Long-term amplexus is primarily maintained by the male's pair of posterior claspers, which is significantly larger than the anterior claspers. The posterior claspers always attach directly to the female's opisthosoma, just lateral to the terminal spines, but the anterior claspers attach in various anteriorward positions on the lateral edges of the female's opisthosoma. We conclude that the mating system of T. tridentatus is fundamentally similar to the American horseshoe crab (Limulus polyphemus), despite the >100 million years of isolation between the two groups. T. tridentatus morphologies, however, show more adaptations to long-term amplexus than those of L. polyphemus.