Although the inclusion of genetically modified (GM) plants in diets fed to fish is a contentious issue, there are few empirical data. The present study addressed nutritional value and potential risks of four maize types (two traditional and two GM maize varieties) and two soy types (one traditional and one Roundup Ready® soy) included at moderate levels in diets fed to Atlantic salmon parr (initial mean weight ± SD; 0.21 ± 0.02 g) during the first 8 months of feeding (March to October), which included the parr-smolt transformation. The GM-maize varieties [Dekalb 1 (D1) and Pioneer 1 (P1)] were hybrids of traditional maize variants [Dekalb 2 (D2) and Pioneer 2 (P2)] and the GM maize MON810®. Four maize diets, two soy diets and one Standard fishmeal-based diet were formulated and fed to fish in triplicate. The maize diets were formulated with 121 g kg-1 GM maize (P1 and D1) or 121 g kg-1 of the traditional untransformed line (nGM-maize, P2 and D2) and the soy diets were formulated with 125 g kg-1 GM soy or 125 g kg-1 of the traditional untransformed line (nGM-soy), all of equivalent nutrient composition. All diets supported good growth and showed no evidence of diet-related mortality. Based on samplings every 6th week, growth was within the normal range and at conclusion of the study body weight did not differ among any of the treatments (range 101?116 g). Besides minor differences on heptatosomatic index (HSI), plasma triacylglycerol (TAG) values and thermal growth coefficient (TGC), body composition, relative organ weights, plasma nutrient concentrations and enzyme activities did not vary among treatments at any sampling. The present findings indicate that the inclusion of GM plants at the given level in salmonid diets poses little, or no, adverse risk to the health of first feeding Atlantic salmon parr and promote normal growth. The paper presents the production related data of this feeding study. Results regarding structure and function of intestinal segments and intestinal organs are presented in Sanden et al.