Two experiments were conducted to determine apparent ileal and fecal P digestibility values in barley, wheat, peas and canola meal and to examine whether the apparent P digestibility values measured in single feed ingredients are additive when used in diet formulation for growing-finishing pigs. In both experiments, five barrows, with average initial body weights of 40 and 35 kg, were fitted with a simple T-cannula at the distal ileum and fed five diets according to a 5 x 5 Latin square design. In exp. 1, the barrows were fed five diets (diets 1, 2, 3, 4, and 5) formulated to contain five graded levels of barley (0, 22.5, 45.0, 67.5 and 90%, respectively) and canola meal (42.7, 36.6, 30.5, 24.4 and 0%, respectively). In exp. 2, the barrows were fed five diets (diets 1, 2, 3, 4, and 5) formulated to contain five graded levels of wheat (88.5, 66.4, 44.3, 22.1 and 0%, respectively) and peas (0, 16.8, 33.6, 50.4 and 67.1%, respectively). Chromic oxide (0.4%) was used as a digestibility marker in both experiments. The pigs were fed twice daily, at 0800 and 2000. The dietary allowance was 1800 g d(-1). Each experimental period lasted 8 d. Representative fecal samples were collected during days 5 and 6. Ileal digesta were collected for a total of 24 h, at 2-h intervals, during days 7 and 8. The ileal and fecal P digestibility values in barley, wheat, peas and canola meal were associated with large standard errors, suggesting a large variability within each feed ingredient. There were no differences (P > 0.05) between the directly determined and predicted apparent fecal P digestibility values in diets 2, 3 and 4 of exp. 1. However, there were differences (P < 0.05) between the directly. determined and the predicted apparent fecal P digestibility values in diets 3 (46.5 +/- 1.0 vs. 38.7 +/- 0.6) and 4 (48.2 +/- 1.2 vs. 41.6 +/- 1.2) in exp. 2, indicating that apparent fecal P digestibility values are not always additive in diet formulation. It is concluded that apparent P digestibility values are variable and not very reliable in diet formulation for swine.