Naturally occurring stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) can be used to differentiate pelagic and benthic prey items and to characterize the trophic position of aquatic organisms. The isotopic signatures of age-0 smallmouth bassMicropterus dolomieufrom six broods in Lake Opeongo, Ontario, were tracked between June 18 and July 24, 1995. Posthatch embryos (4–5 mm in total length) had elevated δ15N values (9‰) that were attributed to the parental origin of their nitrogen pool. The δ15N decreased rapidly, approaching 2‰ for 15-mm smallmouth bass; this complete dilution of the parentally derived δ15N pool corresponded with metamorphosis from larvae into juveniles. The dramatic decline in δ15N provided an opportunity to model the relative importance of somatic growth and tissue turnover in isotopic shifts; tissue accumulation (from exogenous feeding) accounted for 86% of the observed decline in δ15N. Nitrogen isotopes indicated a dietary shift and an increase in trophic position between 17 and 46 mm. By the final sampling date (July 24), body size of age-0 fish ranged from 38 to 46 mm; a positive relationship between δ15N and body size suggested that intrapopulation trophic differences may be responsible for the observed variation in body size. The δ13C values of premetamorphosis (<15-mm) smallmouth bass (−23.2‰ to −26.1‰) were generally higher than adult δ13C values (−25.0‰ to −28.4‰). The δ13C of juvenile smallmouth bass increased with body size (from −24‰ to −21‰), indicating a dietary shift from a mix of benthic and pelagic prey towards reliance on benthic food items.