The relative weight (Wr) index is commonly used to assess fish condition. However, little is known about the relationship ofWrto physiological condition. Whole-body proximate analysis quantifies the general chemical composition of fish but is impractical and too costly for large-scale application to natural populations. Relative weight may offer an alternative method to estimate body composition. We raised juvenile striped bassMorone saxatilisand hybrid striped bassM. saxatilis♀ ×M. chrysops♂ under controlled conditions for a 12-week period and then determined their proximate compositions. Analysis of relationships betweenWrand proximate components indicated thatWrmay be used for estimating body composition and gross energy in juvenile striped bass and hybrid striped bass;Wrwas correlated with percent crude fat, crude protein, ash, visceral fat, and ash-free dry-weight gross energy. Additionally,Wrwas correlated with relative growth and the change in total length for the experimental period. Reserve energy (visceral fat) predicted fromWrmay provide a measure of overwintering fitness and suitability of juvenile striped bass and hybrid striped bass for stocking.