The copulatory behavior of P. trowbridgii is described. The process is categorized sequentially as follows: (1) chelae contact, (2) seizure, (3) turning over, (4) mounting, (5) erection and locking of gonopods, (6) spermatophore deposition, (7) unlocking and recession of the gonopods, (8) release of the female, (9) dismounting, and (10) postcopulatory grooming by the female. Categories 2–9 are male activities. The male may swing one of the fifth pereiopods across the body to support the erected gonopods. Copulation lasts from about 10 to 20 min. Most spermatophores are deposited on the basal segments of the fourth and fifth pereiopods and on the fused sterna between them. Sexual recognition appears to be based primarily on the outcome of an encounter–response behavioral mechanism involving lowered aggression in receptive females but chemosensory information may be involved. Aggressive encounters during the breeding season are indistinguishable from the general fight pattern.