Organic amendment exerts well-known beneficial effects on the physico-chemical properties of soils by providing organic matter and nutrients, thus contributing substantially in maintaining and/or improving soil fertility. Recently, composts and their water-soluble and humic fractions have been shown to be also able to reduce damages from soil-borne phytopathogenic microorganisms, such as fungi of various genera. This capacity may thus contribute to reduce the use of chemical fungicides with consequent economic and environmental advantages. The objective of this study was to evaluate the effects of the humic acid and fulvic acid fractions of a compost (C-HA and C-FA, respectively) on the mycelial growth of two common phytopathogenic soil-borne fungi and two antagonistic fungi resident in soil. The C-HA and the C-FA used were isolated from a compost obtained from a mixture of an olive-oil-mill-wastewater sludge with tree cuttings (58/42, w/w). The phytopathogenic fungi, Fusarium oxysporum f. sp. melonis (FOM) and F. oxysporum f. sp. lycopersici (FOL), and the antagonistic fungi, Trichoderma viride (TV) and T. harzianum (TH), were isolated from soil and grown on potato dextrose agar (PDA) in Petri dishes under laboratory conditions. Aqueous solutions of PDA (4%, w/v), and water-agar (1%, w/v) only for FOM, were steam sterilized and cooled at about 55°C before adding C-HA or C-FA at concentrations of 0 (control, only PDA or water-agar), 10 and 100 mg/L. After pouring of each medium in Petri dishes, cooling and solidification, the fungus was inoculated in the center the plate. Petri dishes were kept in a refrigerated incubator in the dark at a constant temperature of 20°C. The whole experiment was conducted under sterile conditions, and replicated twelve times for FOM and FOL and eight times for TV and TH. After appropriate time periods, and until the fungi reached the border of the plate (about 312 h for FOM and FOL, 96 h for TH and 72 h for TV), the apparent morphology and radial growth of the mycelia were evaluated. All data obtained were statistically treated by one-way analysis of variance and the least significant differences (LSD) test. No morphological changes were observed, with respect to the control, for the four fungi examined as a function of the humic compound added and its concentration. Statistical treatment of data of radial mycelial growth showed that at each evaluation time the growth of FOM was significantly depressed by C-FA and, especially, C-FA at both concentrations. The greatest growth reduction (about 60% of the control) was measured after the first 12 h from inoculation with C-HA at 10 mg/L. The presence of C-HA or C-FA, especially at the higher concentration, caused a significant reduction of FOM growth, with respect to the control, also in sub-optimal nutrient conditions (water-agar). On the contrary, C-HA did not affect significantly, and C-FA slightly promoted the mycelial growth of FOL. These results suggest a specific suppressive action of HA and FA fractions of the compost for the two pathotypes of the same fungus species. In the experiments with the two antagonistic fungi, C-HA at both concentrations and C-FA at the higher concentration caused a highly significant enhancement of mycelial growth of TV along the evaluation period. A slight increase of the mycelial growth of TH was observed in the treatments with C-HA and C-FA, respectively at the lower and the higher concentration, whereas no effects were observed in the other treatments. In conclusion, the HA and FA fractions of the compost can, on one hand, suppress the growth of soil-borne pathogenic fungi, and, on the other hand, exert a beneficial or no influence on the growth of the two antagonistic fungi resident in soil. These effects may, in turn, influence differently the performance of various crops with interesting and promising economic and environmental implications. Keywords: humic acid, fulvic acid, compost, plant pathogenic fungi, antagonistic fungi, Fusarium oxysporum, Trichoderma viride, Trichoderma harzianum