The effect of fungicides and the antagonistic fungus Trichoderma harzianum (Th) on the potato leak pathogen Pythium aphanidermatum (Pa) was investigated in vitro. Rot was reduced by 73% to 89% when tubers inoculated with Pa were treated by immersion in solutions (0.5% w/v) of the fungicides Ridomil MZ58, Dithane M45, Manebe 80 and Tachigaren 360. Complete protection was achieved by dusting inoculated tubers with Ridomil MZ58 diluted in kaolin at a final concentration of 0.1% (w/w). The biological treatments of dipping inoculated tubers in a conidial water suspension of Th at 108 conidia/ml or of dusting with a mixture of a barley culture of Th and kaolin to a final concentration of 2x109 conidia/g were as effective as the chemical treatments. Control was still effective when inoculated tubers were treated with Ridomil MZ58, Tachigaren 360 or the conidial suspension of Th after 24 h incubation at 25 °C.