Signal crayfish Pacifastacus leniusculus were studied in Lake Billy Chinook, Oregon, during 1994 and 1995. Because little was known about the crayfish population, this study was conducted to obtain reliable estimates of life history and population parameters, document historic commercial harvests, and make management recommendations. Crayfish were captured with baited traps and by hand using SCUBA gear. Maturation of both male and female crayfish occurred during the fall of their third year at age 2+. Copulation of mature crayfish began during the first week of October in 1994 and 1995. In 1995, hatching began during the second week of April. The estimated mean days and thermal units required for egg incubation was 166 days and 2,208 degree-days, respectively. Mean pleopod fecundity during the incubation period was 105 � 12 (mean � 95% CI) eggs. Crayfish were captured as deep as 100 m, but 98% of the population was found at depths [less than or equal to] 70 m. The peak relative abundance of crayfish occurred at 10-20 m. Diel activity of crayfish was primarily nocturnal and was skewed towards sunset. Eight age classes were identified from analyses of length-frequency distributions. The sex composition of crayfish appeared to be a 50:50 ratio and only deviated because of behavioral changes related to hatching. The mean density of crayfish, estimated from transects, in five habitat types ranged from 0.24 crayfish/m� to 1.13 crayfish/m�. The trappable population in Lake Billy Chinook was estimated to be 8,437,029 � 2,252,332 crayfish. Total abundance, which included quadrat estimates for 0+ and 1+ crayfish, was estimated to be 35,940,145 � 8,127,159. The mean molt increment of recaptured crayfish was 3.0 mm. The calculated population instantaneous mortality rate was Z=0.67. Total estimated annual production and total biomass of crayfish was 21.16 g/m� and 33.80 g/m�, respectively. The majority of tagged crayfish moved <500 m; however, 21% moved> 1,000 m in one year. Of the total volume of smallmouth bass Micropterus dolomieu stomachs that had food items, crayfish represented 95.3%. The crayfish population in Lake Billy Chinook had been commercially harvested since 1970. Harvest peaked in 1987 at 69,967 kg. There was a strong correlation between the number of commercial licenses and kg of crayfish harvested from 1981 to 1995. Management recommendations include decreasing the commercial fishing season by 2 months and requiring trap-set information to be recorded with harvest sales tickets. If managed wisely, P. leniusculus in Lake Billy Chinook could continue to support a sustainable fishery.