We examined the influence of the oligosaccharide component of a lupin (Lupinus angustifolius) meal when fed to rainbow trout. Two strategies for examining the influence of oligosaccharides were undertaken, including the use of an exogenous enzyme supplement of α-galactosidase to degrade the oligosaccharides and ethanol extraction processing to remove the oligosaccharides. Controls for both processing and degradation of the galactosides were also included in the study. The test treatments were fed to the fish as 30% of their diet and were compared against a reference diet using standard diet-dilution digestibility techniques. The removal of the ethanol-soluble component of the lupin meal had the greatest influence on its apparent digestibility, with significant effects clearly seen on the digestibility of the nitrogen, organic matter and nitrogen-free extractive (NFE) components. Addition of exogenous α-galactosidase to the lupin meal produced a significant difference in the digestibility of nitrogen and numerical improvements in the values of most nutrient parameters. Comparison of the digestibilities of the enzyme-supplemented lupin meal, the galactose-supplemented extracted lupin meal and the original lupin meal treatments indicated that rainbow trout can absorb the monosaccharides produced from the degradation of α-galactosides. This improvement was further validated by examining the influence of the ethanol extraction process, which showed that it is the removal of the ethanol-soluble components, most likely oligosaccharides, that is the contributing factor. These findings indicate that further processing of lupin meals to remove their oligosaccharide fraction would have beneficial nutritional implications for fish.