A digestibility trial was conducted to investigate the effect of varying the incorporation levels of herring meal into the test diet on the apparent digestibility coefficients (ADCs) for protein and energy and to compare the values calculated using two different published equations [Cho, C.Y., Slinger, S.J., Bailey, H.S., 1982. Bioenergetics of salmonid species: energy intake, expenditure and productivity. Comp. Biochem. Physiol. 73B, 25–41; Forster, I., 1999. A note on the method of calculating digestibility coefficients of nutrients provided by single ingredients to feeds of aquatic animal. Aquacult. Nutr. 5, 143–145.]. Herring meal was incorporated at levels of 10%, 20%, 30%, 40% and 50% to 90%, 80%, 70%, 60% and 50% (w/w basis) of the reference diet, respectively. Haddock with mean initial body weight of 49 g were randomly distributed to each of six tanks (40 fish/tank). The reference and test diets were fed to fish to apparent satiation three times daily during the week and twice daily during weekends. Fish were acclimated to the experimental diets for 1 week and fecal collections were made for 3 consecutive weeks. Feces collected in the same bottle from each tank for 5 consecutive days were used as one replication for the treatment. The ADCs for protein and energy of the reference diet were 94.8% and 90.6%, respectively. Neither increasing herring meal incorporation from 10% to 50% into the test diet nor the method of ADC calculation had any significant effect on protein digestibility of the feed ingredient, which remained constant at averages of 93.7% (Cho) and 94.0% (Forster). The same was true for energy digestibility with averages of 95.4% (Cho) and 95.5% (Forster).