The effects of a mineral block for horses on in vivo digestibility and in vitro fermentability with equine fecal inoculum were evaluated. Fifty healthy horses from three groups (lactating mares n = 19, working horses n = 18, and maintenance horses n = 13) were randomly assigned to two treatment groups (with or without the mineral block; Ca 10.0%, P 12.0%, Zn 12.1 mg/kg, Cu 2,050 mg/kg, Mn 4,050 mg/kg, Se 30 mg/kg, and I 105 mg/kg). Dry matter digestibility was estimated with an internal marker. Samples of diet were incubated with equine fecal bacteria with varying amounts of mineral block (0, 1.1, 3.6, and 6.2 mg/g dry matter [DM]) to record gas production and to estimate in vitro DM digestibility. The results showed that mineral supplementation with the blocks increased in vivo DM digestibility (P < .01) in all groups, but there was an interaction (P < .01) with a greater response in the maintenance horses (55.5% vs. 78.0%) compared to lactating mares (62.8% vs. 79.6%) and working (70.3% vs. 75.1%). Block consumption was lowest in the lactating mares (12.8 g/d), intermediate in the working horses (44.6 g/d), and highest in the maintenance horses (74.2 g/d). The mineral supplementation did not affect the kinetics of gas production but tended (P = .10) to improve the in vitro DM digestibility (37.01% vs. 38.34%). Mineral block supplementation increased dry matter digestibility in horses. The unsupplemented control diet was deficient in several minerals, and block intake was not proportional to the mineral requirements.