This research evaluated the effects of the mean particle size (MPS) of soybean meal (SBM) and the inclusion of fiber in the diet on growth performance, total tract apparent retention (TTAR) of nutrients, and gastrointestinal tract (GIT) development in broilers from 0 to 42 d of age. The experiment was completely randomized with 8 treatments organized as a 2 × 4 factorial with 2 MPS of the SBM (coarse; 1,385 μm vs. micronized; 170 μm) and 4 diets that varied in the source of fiber used [control vs. 3% of oat hulls (OH), soy hulls (SH), and sugar beet pulp (SBP)] as main effects. The experiment had 2 feeding phases (0 to 21 and 22 to 42 d), in which the control diets (based on rice and SBM) varied exclusively in the MPS of the SBM used (coarse vs micronized). The experimental diets resulted from the dilution (wt:wt) of the control diets with 3% OH, SH, or SBP, respectively. Each treatment was replicated 6 times and the experimental unit was a floor pen with 20 chicks. Growth performance [(BW gain, feed intake (FI), and feed conversion ratio (FCR)] data are presented by feeding phase and cumulatively. TTAR of DM, OM, CP, and the AMEn of the diets was determined at 21 d of age. GIT traits were evaluated at 21 and 42 d of age. Data were analyzed as a completely randomized design with MPS of the SBM and fiber source inclusion as main effects, using the MIXED procedure of SAS. From 0 to 42 d of age, FCR was better for broilers fed micronized SBM than for broilers fed coarsely ground SBM (P < 0.05). Particle size of the SBM did not affect nutrient digestibility. In relative terms (% BW), the full proventriculus and gizzard were heavier in birds fed the coarse SBM at both ages (P < 0.05). At 42 d of age, gizzard pH was lower for the coarse than for the micronized SBM (P < 0.05). Fiber inclusion increased FI (P < 0.05) but did not affect energy conversion ratio. Birds fed OH ate more feed than birds fed the control diet, with birds fed SH and SBP being in an intermediate position (P < 0.05). Fiber inclusion did not affect nutrient digestibility. In relative terms (% BW) the inclusion of extra fiber in the diet increased the weight of the empty gizzard, an effect that was more pronounced with OH than with SH or SBP at both ages (P < 0.05 for the interaction). Also, irrespectively of the source, fiber inclusion reduced gizzard pH at both ages (P < 0.01).