Freeze‐dried or oven‐dried antarctic krill (Euphausia superba) and southern blue whiting (Micromesistius australis) (SBW) were analysed for their proximate and amino acid contents. Each sample was then combined into semi‐purified diets and fed to male weanling rats. The krill and SBW meal provided the sole source of protein in each diet. The true digestibility of the protein of all diets was consistently high (94.0–96.6%). The mean biological value (BV) of the SBW meal was 78.7, similar to that of white fish meal (81.0). The krill meal had significantly lowerBV(mean 65.9), which could be significantly improved on addition of 0.1% DL‐methionine (BV 75.6) or 0.1% cystine (BV72.9).