The growth efficiency of juvenile Atlantic halibut, Hippoglossus hippoglossus (L.), was investigated by comparing wet weight specific growth rates (SGRs) over 28 days with protein synthesis rates measured using a non-invasive stable isotope (15N) tracer technique. A diet containing 15N-labelled protein was fed at a single meal and individual feed intake was measured by X-ray radiography to allow calculation of the amount of 15N consumed. Excreted ammonia and urea were collected from each fish and the ratio of 15N to 14N was used to calculate rates of protein synthesis. Peak rates of ammonia excretion occurred 12 h post-feeding. Rates of urea excretion did not change after feeding and remained relatively constant over the 48-h measurement period. Urea accounted for 17% of the measured nitrogen excretion and showed no enrichment with 15N. Mean protein growth rates were 1.31 ± 0.06% day-1, while whole animal fractional rates of protein synthesis were 2.02 ± 0.24% day-1. The mean protein synthesis retention efficiency was 77.41 ± 9.09%, which is higher than that recorded for most other teleosts. This suggests that halibut have a relatively low cost, high growth efficiency growth strategy.