Summary In laboratory, glasshouse and field experiments, Pythium oligandrum has been identified as a mycoparasite capable of suppressing damping-off pathogens in emerging sugar-beet. Furthermore, in paired cultures with P. oligandrum on agar-plates, the fungi Botrytis cinerea, Acremonium strictum, Acremonium apii, Paecilomyces sp., Penicillium albidum, Phialophora malorum, Scopulariopsis brevicaulis and Humicola fusco-atra, were also distinctly inhibited by P. oligandrum. A smaller inhibitory effect of P. oligandrum was observed on Acremonium vitellinum, Gliocladium roseum and Chaetomium globosum. On the other hand, the fungi Mucor heterosporum, Rhizopus arrhizus and Mortierella sp. were found to be inhibitors of P. oligandrum, although unable to stop its growth. Drechslera sp. (conidial stage of Cochliobolus sativus) and Mucor piriformis proved more effective, entailing the disappearance of the mycoparasite in mixed cultures. However, the inhibitory activity of those fungi, demonstrated on agar plates, was not confirmed in vivo. The inoculation of seeds with mycelium of P. oligandrum + P. ultimum with and without addition of M. piriformis, led us either instance to an increase in both the rate of emergence, and the weight and state of health of the plants involved. In a pot trial, an emergence rate of sugar-beet plants higher than achieved when using a chemical called Agronex Hepta T 30 was attained after the sugar-beet seeds had been treated with a biopreparation of P. oligandrum in liquid form. But the liquid preparation had only a short-lived effect. In powdery form, the biopreparation was found to be more suitable in that respect. The current method of producing of the latter has made it possible to stabilise the component of P. oligandrum oospores without impairing their vitality and infectibility, even after three years' storage. An unusually high production of oospores has been obtained by means of surface stationary cultivation of the mycoparasite and a new procedure to prepare the agent in powdery form. Concentration of the final product varied within the range of 50 to 500 million oospores/g.