Fillets from Walleye pike (Stizostedion vitreum) and Northern pike (Esox lucius) captured from the Upper Wisconsin River were analyzed for volatile components using simultaneous steam distillation-extraction and GC-MS techniques. Based on comparative odor assessments of eluting GC fractions and GC-MS analysis between control and tainted fish, trace amounts (ppb) of alkylphenols (2-isopropyl-, 4-isopropyl-, 2,4-diisopropyl-, 2,5-diisopropyl-, 2,6-diisopropyl-, 3,5-diisopropyl-, 5-methyl-2-isopropyl-, and 2-methyl-5-isopropyl-) and thiophenol were identified as the principal contributors to flavor-tainting found in fish during the spring of the year. Geosmin and 2-methylisoborneol were major contributors to mustiness flavor-tainting occurring in fish captured in the late summer and were more abundant in downstream fish than in upstream control fish.