This research tested the effects of paper birch, Betula papyrifera Marshall, condensed tannin on larval performance of the whitemarked tussock moth, Orgyia leucostigma (J. E. Smith). We conducted laboratory bioassays on fifth stadium larvae. Larvae were reared on one of three diets: control (no condensed tannin), moderate condensed tannin (8.8% dry mass), and high condensed tannin (17.6% dry mass). Although survivorship was not different between the treatments, larvae fed diets amended with condensed tannin exhibited increased stadium duration, decreased relative growth rate, and decreased food conversion efficiencies. Prolonged development times enabled larvae to compensate for low consumption and growth rates such that insects on tannin diets ate more and grew larger than insects on the control diet. Analysis of tannin levels in food, frass, and body tissue indicated that larvae do not metabolize condensed tannin, but concentrate and egest it. Our results show that paper birch condensed tannin has both positive and negative effects on the performance indices of whitemarked tussock moths. However, whether the benefits of increased final size (and possibly fecundity) outweigh the risks of increased development time and prolonged exposure to natural enemies remains unclear.