Knight, J. D. 2012. Frequency of field pea in rotations impacts biological nitrogen fixation. Can. J. Plant Sci. 92: 1005-1011. Economic, environmental and energy concerns about the use of nitrogen (N) fertilizers in crop production have prompted the examination of increasing the frequency of pulses in crop rotations to capitalize on biological nitrogen fixation (BNF). Plots from a field experiment established in 1998 at the Agriculture and Agri-Food Canada Research Farm at Scott, SK, were sampled in 2008, 2009 and 2010. Rotations that included pea every year (continuous pea), every second year (pea-wheat), every third year (pea-canola-wheat) and every fourth year (canola-wheat-pea-wheat) were evaluated for BNF using the enriched ¹⁵N isotope dilution technique. Nitrogen from BNF in the seed and straw, total above-ground N, seed and straw yield and soil available N and P were evaluated. In 2 of 3 yr, the highest BNF occurred in the two most diverse rotations. Continuous cropping of pea resulted in drastically low BNF in 2008 and 2009. Nitrogen derived from atmosphere in the continuous pea was 15% compared with an average of approximately 55% across all other rotations in these 2 yr. The reduction in BNF was not due to lower productivity in the continuous pea rotation, nor from higher initial soil inorganic N levels inhibiting BNF. In the third year of the study (2010), the more than double the normal precipitation received during the growing season stimulated BNF in pea in the continuous pea rotation. Determining whether the rotation effects on BNF are due to N mineralization of the previous years' crop residues requires further investigation.