The objectives of this study were to evaluate seasonal changes in the energy density of age-0 and age-1 muskellungeEsox masquinongyand to compare energy density to various estimates of condition. Three treatment groups of muskellunge were evaluated to determine temporal changes in energy density (J/g wet weight [ww]), water content, condition factor (KTL;K=W/L3, whereW= weight andL= total length in centimeters), and relative weight (Wr.; ratio of actual to “standard” weight) through the first year following hatching. Treatment groups were (1) hatchery (muskellunge reared and maintained in hatchery ponds), (2) stocked (hatchery-reared muskellunge stocked in lakes), and (3) natural (muskellunge naturally produced in lakes). Energy levels and relative condition were compared for fish 4 and 11 months old. Differences in energy density were observed between all three treatment groups. An average overwinter reduction of 494 ± 192 J/g ww in energy density was observed over all treatment groups combined. Natural fish lost less energy (8%) over winter than either hatchery (12%) or stocked fish (15%). A simple linear model effectively relates energy to indices of condition for muskellunge. A weak positive relation (P< 0.0001,r2= 0.39, 0.40, and 0.43) was observed between dry weight energy density and three indicators of fish condition (percent water,K, andWr). Our results show that condition indices may not be the best indicators of seasonal fluctuations in total energy within and between fish populations. Seasonal fluctuations in energetic values for a population can be more accurately determined through assessment of percent water in individual fish.