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Pulse crops, including lentil (Lens culinaris Medik.) and pea (Pisum sativum L.), can improve the sustainability of Northern Great Plains cropping systems, largely through biological N fixation. Greater N fixation amounts can help producers to increase yield while decreasing N fertilizer inputs for the following crop. There may be potential to breed greater N‐fixing pulse varieties, yet little is known about varietal differences in N fixation. Nitrogen fixation of pea and lentil varieties was quantified at two sites in Montana from 2019 to 2021 using an N difference approach and the 15N natural abundance method. Riveland and CDC Richlea were frequently high N‐fixing lentil varieties, both fixing ca. 130 kg N ha−1 in the site‐year with the most favorable growing conditions. No pea variety had consistently greater N fixation than others, despite N fixation ranging from 88 to 135 kg N ha−1 in one site‐year among varieties. Nitrogen fixation by lentil had an inverse relationship with days to flowering but was not correlated with days to maturity. Nitrogen fixation by pea was positively correlated with days to maturity but was not correlated with days to flowering. Breeding lentil and pea for high N fixation by selecting high N‐fixing varieties is likely difficult, as varieties performed differently under variable environmental conditions. Breeding efforts based on traits, such as days to flowering, could be more successful. There were more positive correlations between N fixation parameters and seed yield for pea than for lentil, suggesting that breeding for greater yields could increase N fixation for pea but not lentil. Core Ideas: Variation in environmental conditions strongly influenced N fixation by lentil and pea varieties.Differences in N fixation among varieties were up to 21 kg N ha−1 for lentil and 47 kg N ha−1 for pea.Two lentil varieties often fixed more N than others, while no pea varieties performed consistently well.Breeding lentil for shorter days to flower could increase N fixation, suggested by an inverse relationship.Breeding pea for greater yield or longer days to maturity could increase N fixation, suggested by positive relationships. [ABSTRACT FROM AUTHOR]