Previous research on late-season N fertilization of pecans [Carya illinoinensis (Wangehn) K. Koch] has shown significant uptake and storage of N in perennial tissues (roots, trunk, and shoots) that was used in subsequent years. The objectives of this study were to follow the fate of 15N applied at three different stages during pecan kernel fill in both the soil and tree components. In August and September 2002, 15N-labeled ammonium sulfate (9.94% 15N atom excess) was applied (56 kg N/ha) to nine pecan research trees during the early [3 days into kernel fill (DIK)], middle (25 DIK), and late (38 DIK) stages of pecan kernel fill near Las Cruces, N.M. In November 2002, about 67% of applied 15N was recovered from the soil and 13% from tree components. More 15N was recovered in nuts from the early treatment than middle or late treatments. Recoveries for May 2003 were 27% and 60% for tissues and soils, respectively. Leaf recovery increased an average of 14% in May 2003 over November 2002 leaves. More 15N was recovered from the late treatment in all tree components for May 2003 than early or middle treatments. The primary source of N for spring growth was 15N stored in perennial tissues. Fifteen months after 15N fertilizer was applied during kernel fill in 2002 about 24% remained in the soil, 28% had been used by the tree, and 48% was lost to the environment.