Biocontrol activities in the oospore suspensions of 17 isolates of Pythium oligandrum (PO) were examined for their effect against post-emergence seedling diseases. The seeds of sugar beet that had been commercially chemical-pelleted to prevent pre-emergence damping-off were sown in Aphanomyces cochlioides-infested soil on trays. PO isolate MMR2 proved most effective in controlling post-emergence damping-off, at a level equivalent to that of applying fungicides. Oospore suspensions stored for 188 days or 379 days at 4°C were as effective against A. cochlioides post-emergence damping-off as chemical control. Treating soil with PO oospores by mixing suspensions into the surface of soil in paper pot nurseries or by drenching seedlings 32 days before transplanting could control Aphanomyces root rot in the field without additional applications. The control effect conferred by PO suspensions was also demonstrated in vitro. A polymerase chain reaction (PCR) assay used to detect PO indicated that this oomycete could colonize the rhizosphere of sugar beet plants cultivated in the field when applied before transplanting. These results suggest that PO has the potential to control both post-emergence damping-off and root rot caused by A. cochlioides.