Population biology and acid tolerance of the crayfish Cambarus bartonii bartonii (Fabricius) from southern Appalachian Mountain streams were investigated. Field studies were conducted primarily from May through October, 1985 at Coweeta Hydrologic Laboratory (U. S. Forest Service Southeastern Experimental Station) in Otto, North Carolina, to describe selected population parameters and life history events for this species in two first-order streams. Laboratory experiments were conducted using crayfish from Coweeta and from Craig Creek (Jefferson National Forest, Montgomery County, Virginia) to determine median lethal concentrations (LC₅₀) for sulfuric acid exposure, and to investigate physiological effects, influence of ambient water temperature, and effects of episodic acid events on molting. Population estimates of crayfish ranged from 210 to 683 crayfish in 20 to 25m study sections (≃9.2 to 29.8 per m²) of Ball Creek and Pinnacle Branch. Mean catch per baited funnel trap and mean catch per hour of hand collecting were determined in three study sections of each stream in May and September to obtain relative abundance estimates and compare capture methods. Each method selected for different segments (size classes) of the population. Sex ratios were balanced (1:1) over the whole sampling season, but differed significantly from 1:1 in some months, probably due to reproductive activity. Sexually mature males and females (form I males, females with full cement glands) were observed in all months sampled. Acute (96h) laboratory lethality tests yielded LC₅₀ values for pH's of 2.43, 2.56, 2.85 and 2.43 for Coweeta intermolt adults (25.3 to 43.2mm carapace length, CL), late juveniles (11.5 to 19.9mm CL), early juveniles (3.9 to 12.8mm CL) and Craig Creek intermolt adults (23.0 to 41.3mm CL), respectively. Decreased water temperature resulted in increased acid tolerance of intermolt adults (LC₅₀ = pH 2.33), and increased survival time during severe acid exposure. Acid exposure of intermolt adults in relatively soft water affected internal ion regulation, causing decreased hemolymph [Na⁺] and increased [Ca⁺⁺]; no Ca⁺⁺ mobilization (loss) from carapaces was observed. Preliminary work showed that extreme acidification had visible adverse effects on molting adults, causing failure of exoskeletons to become rigid. Episodic acid events at Coweeta do not appear to be a direct mortality problem to C. b. bartonii at this time. However, this does not preclude the possibility of future problems, particularly sublethal effects to reproductive activity or early life history stages, if increasing acidification and gradual loss of soil buffering capacity of watersheds persists. / M.S.