The goal of this project was to quantify the background physiological condition of mussels in the Minnesota and St. Croix River basins using glycogen as an indicator of physiological stress. We obtained 230 samples of foot tissue from Lampsilis cardium (n=118) and Lasmigona complanata (n=112) from a total of four rivers during September 1-5, 2015. From each sample, we weighed out a 10.0 ± 2.0 mg aliquot for glycogen determination by the alkaline digestion and phenol-sulfuric acid spectrophotometric method. The accuracy of glycogen determinations was quantified by the use of procedural blanks, replicates of an in-house reference material, triplicate analysis of four aqueous calibration standards, duplicate analysis of 48 analytical samples, triplicate analysis of 23 randomly selected tissue samples, and triplicate analysis of standard additions.