Perennial grasses grown for bioenergy production can maintain high yields for many years without the need for nitrogen fertilizer. Consequently, it is suspected that these crops are capable of fixing atmospheric nitrogen. Miscanthus x giganteus is a suspected nitrogen fixer due to the presence of nitrogen fixing bacteria in its tissues and reports of a positive nitrogen budget. The objective of this study was to evaluate the nitrogen fixation contributions to miscanthus and switchgrass, two perennial bioenergy feedstock crops, by implementing three different methods:  ¹⁵N natural abundance technique, acetylene reduction assays (ARA), and the nitrogen balance technique.  A thorough accounting of N in these crops showed a positive N balance of 29 kg N ha^-1 for miscanthus, which could indicate N additions from biological nitrogen fixation.  Nitrogenase activity, as detected by ARA incubations, is an indicator of nitrogen fixation. Nitrogenase activity was detected for both miscanthus and switchgrass; however, the level of activity appears negligible when compared to measurements of soybean.  Similarly, initial results show little difference in ¹⁵N natural abundance of miscanthus and switchgrass compared to reference plants. These perennial bioenergy feedstocks may be capable of fixing small amounts of nitrogen; however, we found no evidence they are capable of fixing significant amounts of nitrogen.  While high nitrogen fixation rates would improve the sustainability of these bioenergy feedstocks, they are nonetheless highly efficient at retaining N and a more sustainable choice for bioenergy production with respect to nitrogen.