We compared oxygen consumption (MO2, mg/kg/h) of c. 80 g rainbow trout (Oncorhynchus mykiss) in an intermittent-flow swim respirometer at 15 °C. Before the tests the fish were grown in flow through tanks (15 °C) with either fishmeal (FM) or corn gluten meal (CGM) based diets (c. 52% protein) for a period of 3–4.5 months. Ten individuals from both treatment groups were fasted for 48 h before the swim test, which consisted of 18 loops of 210 s over three different periods: acclimation period (6 loops at 0.5 body lengths per s, BL/s), exercise period (8 loops at increased speed from 1 to 2.5 BL/s with recovery loops at 0.5 BL/s), and a recovery period (four loops at 0.5 BL/s). We did not observe significant differences in MO2 between the two groups at any of the three measurement periods (repeated measures-Anova). The maximum (mean ± SE) MO2 values, measured during the last exercise period at 2.5 BL/s, did not differ significantly between the treatments: 404 ± 18.7 mg/kg/h and 427 ± 50.6 mg/kg/h in FM and CGM groups, respectively. Our result supports an earlier finding that origin of the protein does not affect MO2 during swimming in salmonids. This is the first report of the effect of a plant protein on MO2 of a carnivorous fish during forced swimming, and these data lend support to further development of sustainable diets to replace fishmeal with plant proteins. Abbreviations: BLbody lengths CFcondition factor CGMcorn gluten meal FMfishmeal MO2oxygen consumption (mg/kg/h)