Control of preemergence damping-off caused by Phytophthora parasitica Dastur was investigated on three bedding plant species in a 1 peat: 1 vermiculite medium (v/v) limed at 3 kg·m–3 and drenched with aluminum at 10, 25, or 50 meq Al/100 cm3 medium. Aluminum as Al2(SO4)3 was applied as a drench at 0.75, 1.9, or 3.75 g/150 ml water to the surface of infested medium in 650-cm2 plug trays (1300-cm3 tray volume). All concentrations of aluminum were effective in controlling preemergence damping-off of snapdragon (Antirrhinum majus L.) and vinca (Catharanthus roseus G. Don, Madagascar periwinkle), but only 50 meq Al+3/100 cm3 medium was effective for petunia (Petunia ×hybrida Hort. Vilm.-Andr.). At 4 days after seeding and drenching with aluminum sulfate, exchangeable aluminum was 0, 0.5, and 2.03 meq Al+3/100 g medium, respectively, for the three concentrations used. Control of damping-off of snapdragon and vinca with 10 meq Al+3/100 cm3 medium with no detectable exchangeable aluminum 4 days after application suggests that P. parasitica was suppressed by aluminum early in the host–pathogen interaction, whereas petunia was susceptible to damping-off for a longer period before seedling emergence. Aluminum was not phytotoxic to vinca, snapdragon, or petunia grown in a limed medium.