Three experiments were conducted to evaluate the effects of corn particle size on growth performance, ME(n), apparent total tract amino acid digestibility, and cecal microbial populations when young chicks were fed a corn-soybean meal-based diet. In all experiments, 1-d-old chicks were randomly assigned to 1 of 4 dietary treatments containing corn particle geometric mean diameters of 557, 858, 1,210, or 1,387 microm. Chicks were fed the experimental diets from 0 to 21 d posthatch and ME(n) and apparent total tract amino acid digestibility were determined at 7 and 21 d of age. When compared with the smallest geometric mean diameters of 557 microm, feeding the larger particle sizes had no effect on growth performance from 0 to 21 d in all experiments. Feeding larger corn particle sizes resulted in increases in relative gizzard weights in all experiments, with the greatest increase occurring with the 1,387-microm corn particle size. Gizzard pH was unaffected by corn particle size in all experiments. The ME(n) values and digestibility coefficients for most amino acids were higher (P < 0.05) at 21 d than at 7 d for all dietary treatments. In experiment 2, the ME(n) values showed a significant linear decrease (P < 0.007) as corn particle size increased at 7 d. The digestibility of most amino acids was unaffected by corn particle size at 7 or 21 d. Cecal lactobacilli populations were increased (P < 0.05) when the largest particle size was included in the diet. The results of these studies indicated that feeding larger particle size corn increased gizzard weight significantly but had no consistent effect on overall growth performance, ME(n), or amino acid digestibility.