The aim of this experiment was to compare the D(+) and D(-) chicken lines genetically selected for divergent digestion efficiency by testing the effects of diet particle size on growth performances, digestion efficiencies, and digestive organ weights in both lines. A 2 x 3 factorial arrangement of treatments was used to test the D(+) and D(-) lines (sixth generation) and 3 diets, namely a pelleted standard corn diet ( S), a pelleted hull diet (H) made by diluting S diet with 7% coarse cereal hulls, and a coarse corn diet (C) identical to the S diet, distributed as 30% coarsely crushed corn mixed with the 70% pelleted remaining part. Experimental diets were fed from 7 to 26 d of age. Combining results from all diets obtained at 26 d of age, D(+) birds showed 9% heavier (P < 0.0001) gizzard and 10% lighter (P < 0.0001) small intestine than D(-) birds. The AME(n) and digestibilities of lipids, protein, and starch measured at 3 wk of age were, on average, 3.5, 5.6, 5.8, and 0.5% higher (P < 0.0002) in D(+) than in D(-) birds, respectively. Significant (P <= 0.05) interactions between lines and diets were observed for AME(n) and digestibility values. Measured: calculated AME(n) ratio and digestibilities of protein and starch were improved (P < 0.05) by dietary coarse particles in D(-) birds, not in D(+) birds. Measured: calculated AME(n) ratio differed between lines by 6.0% with S diet versus 2.3% with H and C diets. In D(-) birds, the digestion increases due to coarse particles were associated with increased weights of gizzard (P < 0.0001) and pancreas (P < 0.05). In D(+) birds, coarse particles resulted in increased weights of gizzard (P < 0.0001) only. In conclusion, growth and functions of gizzard and pancreas needed to be stimulated by dietary coarse particles for optimum digestion efficiencies in D(-) chickens, whereas such stimulations were not needed in D(+) chickens. The D(+) chickens showed high digestion efficiencies in all cases, independently of diet particle size.