The Wood River Sculpin, Cottus leiopomus, is endemic to the Wood River basin in Idaho and is a nongame species of concern because of its limited distribution, but the status and biological characteristics of this species were previously unknown. We collected 716 C. leiopomus from ten populations across the Wood River basin from streams with a variety of physiochemical conditions to estimate length and age at sexual maturity and other demographic characteristics. Most C. leiopomus were estimated to be age-1 (16%), age-2 (33%), age-3 (30%), and age-4 (12%); the oldest was estimated to be eight years old. Estimated total annual survival rate was consistent across all study sites, averaging 66% and ranging from 56 to 70%. Survival was positively correlated with mean stream width and negatively correlated with stream gradient, although sample sizes for these comparisons were small and we are cautious in asserting a causative effect. Sculpin reached 60 mm total length by age-2 and reached 100 mm at around age-4; the largest C. leiopomus captured was 121 mm. Sex ratio was near 50:50 for most populations and averaged 51% female across all populations. Fecundity ranged from 38 to 314 eggs and formed a linear relationship with fish length (r²  =  0.67). At the two sites where sampling occurred in the spring and maturity could be more definitively determined, almost all C. leiopomus age-3 and older were mature, regardless of gender. No age-1 fish were mature, and no age-2 males were mature, but 83% of age-2 females were mature. We estimated that females transitioned from immature to mature at about 55 mm and males at about 60 mm. These results are the first published data on the life history characteristics of C. leiopomus and provide useful information for the management and preservation of this species.