The role of microbial flora in the ability shown by poplar bark compost to combat the carnation phytopathogen Fusarium oxysporum f.sp. dianthi was investigated. Compost was divided into two parts, one sterilized and the other not, and was added to two different types of soil naturally infested with Fusarium oxysporum f. sp. dianthi. Experiments were carried out in greenhouse benches. At 15-day intervals the main microbial groups present in the soil were analysed and visual inspections were carried out to evaluate the mortality of the plants. Protection against Fusarium oxysporum f. sp. dianthi due to the compost added was found in both soil types, and the protection was more marked in the case of sterile compost. This difference in protection is due to a different growth rate of the soil zymogeneous fraction, a consequence of the different amounts of easily assimilable organic matter contained in the two types of compost.