Two soil substrates, a sandy loam soil and a peat soil, were artificially infested with Meloidogyne hapla Chitwood. Additional organic material (5, 10, 15 and 20 volume %) in the form of either fermented household waste material (compost) or ground cocoa bean peels, was added. The initial population density of nematodes was counted as L 2 larvae; an estimation of the final population was obtained by counting the typical root knots of the lettuce plants which were grown on the soil substrates. At the same time the microbiological activity of the soil mixtures was determinated by a soil plating method (a selective medium for fungi and a selective medium for bacteria) and by a Warburg technique (CO 2 production). The addition of 10% fermented household waste material to the soil substrate resulted in a minimum number of root knots accompanied by a maximal microbiological activity. This phenomenon indicates a microbial competition more than a direct influence of toxic entities in the compost because of the higher rate of Meloidogyne attack with higher amounts of compost, linked with a decreased number of bacteria and fungi. In the experiment with ground cocoa bean peels, the biological activity (as well as the number of fungi and bacteria) increased with increasing addition of cocoa peel material and the number of root knots decreased with addition of up to 20% ground cocoa peels.