We conducted experiments to determine isotope changes in the deposit-feeding chironomid larvae Chironomus acerbiphilus during feeding, starvation and metamorphosis. Isotope changes in chironomid larvae occurred mainly during growth and rarely afterward. This finding indicates that chironomid isotope turnover mainly occurs in conjunction with growth and suggests that chironomid larvae only break down newly assimilated food for energy during periods of no growth. Chironomid δ13C values significantly increased throughout the starvation experiment, indicating that chironomids preferentially break down components with lower δ13C content during starvation. We found significant changes in chironomid isotope ratios (15N enrichment) during pupation. This evidence suggests that the physiological condition of animals (such as during an active growth phase or pre- or post-molting) is important to their stable isotope ratios. Copyright © 2007 John Wiley & Sons, Ltd.