We evaluated the relative weight (Wr) condition index as a field assessment tool with pumpkinseedLepomis gibbosusand golden shinerNotemigonus crysoleucas, focusing on sources of variability and potential ofWras a predictor of growth, prey availability, and environmental conditions in 10 southern Quebec lakes over 2 years. To allow calculation ofWr, we developed standard weight (Ws) equations for both species, using the regression-line-percentile (RLP) technique. The proposedWsequation in metric units (grams wet weight and millimeters total legnth, TL) for pumpkinseed is log10Ws= –5.179 + 3.237 log10TL; for golden shiner it is log10Ws= –5.593 + 3.302 log10TL. Spatial and temporal variation inWrwas highly significant and largely asynchronous in both species, although spring values were lowest in most lakes. TheWrindex frequently varied with length, prompting us to examine relationships in stock and quality length fish separately. We found little evidence for a relationship betweenWrand growth in either species. PumpkinseedWswere positively correlated with total benthic invertebrate biomass: stock lengthWrwas positively correlated with chironomid biomass, and quality lengthWrwas positively correlated with gastropod biomass. The relative weight of quality length golden shiners was positively correlated with chironomid biomass. Our results and those of other studies suggest that the common assumption of a relationship betweenWrand growth in field populations should be reconsidered, but thatWrcould be cautiously used as a working index of prey availability. We recommend empirical or experimental verification whenWris used as an assessment tool in field populations.