Fecal particle size (FPS) may be an indicator of chewing efficiency and digestive function in horses. The objective of this study was to evaluate the effect of horse age on FPS by comparing FPS measured by wet sieving analysis between 6 mares (12.8 ± 2.8 y) and their yearlings (1.18 ± 0.06 y). Both groups were housed on pasture with access to alfalfa hay and fed 2.73–3.64 kg/d of concentrate feed formulated for breeding and growing horses. Feces were collected by free catch, and divided into subsamples. One subsample was used for dry matter (DM) determination and the remaining samples were frozen until analysis. Before analysis, samples were thawed and duplicate aliquots containing 3.8 g of DM were soaked overnight in 200 mL of water. The amount of sample needed to contain 3.8 g of DM was calculated using the predetermined sample DM. Samples were washed over a set of 203.2 mm stainless steel sieves (VWR, Radnor, PA) with sieve openings of 4, 2, 1, 0.5, 0.25, 0.125 and 0.063 mm using a standardized water flow rate of 6 L/min for 5 min. Each sieve was rinsed clean into a beaker from which particles were filtered onto dried, pre-weighed filter paper for DM determination. Particles passing the smallest sieve were not collected but the amount was calculated by difference between total DM sieved and DM retained on all sieves.Weighted mean FPS and percent DM retained on each sieve as a percent of total DM sieved were analyzed using paired t-test procedure in SPSS V28. There was no difference in weighted mean FPS between mares (1.59 mm ± 0.09) and yearlings (1.47 mm ± 0.06) (P > 0.05). There was no difference in the percentage of DM retained on the 3 largest and 2 smallest sieves between mares and yearlings (P > 0.05). Compared with yearlings, there were trends for mares to have greater DM retained on sieves with diameter 0.5 mm (P = 0.096) and 0.25 mm (P = 0.08). Mean ± SE percent DM retained for mares and yearlings, respectively, was 9.6% ± 1.6 and 6.95% ± 1.2 (4 mm sieve), 12.70% ± 1.09 and 10.65% ± 1.08 (2 mm sieve), and 12.48% ± 0.89 and 12.19% ± 1.7 (1 mm sieve). Soluble DM calculated by difference as DM escaping the smallest sieve was greater in yearling samples compared with mare samples (37.02 vs 29.88%; P = 0.019). The small sample size of this study is a limitation but suggests there may be differences in fecal particle size distribution between mare and yearling pairs that warrant further investigation.