Maternal and offspring behaviors of P. trowbridgi were studied in aquaria. Ovigerous females approaching hatching time were more aggressive than other adult crayfish. At hatching, the female as- sumed a posture with the abdomen well extended and waved the pleopods very actively. Individual broods hatched in 2 days or less and 14 of 16 broods hatched within a 4-day period. Incubation experiments showed that no damage was incurred by eggs removed from the pleopods, and that water circulation was necessary to avoid mass mortality of such eggs. After hatching, the young remained with the parent for 21 to 25 days. From 8 to 12 days were spent in Stage 1 and 11 to 14 days in dependent Stage 2. The second-stage young molted into Stage 3 from 11 to 16 days after independence. Transition to an independent existence involved depletion of food reserves, loss of the clinging reflex, reversed orientation on the pleopods, exploratory and feeding behavior, and the onset of aggression. Independent Stage 2 crayfish avoided each other and their parent. Their response to artificial shelter, and background color and pebbles in relation to light intensity, has survival value in their natural habitat. The critical phases of reproductive biology following mating are: egg-laying, hatching, molting into Stage 2, and exploration. Parental mobility jeopardizes survival of the young and this may account for the maternal female's solitary and aggressive nature, and her inclina- tion to confine herself to excavated dens and similar retreats.