To test the biocontrol potential of Penicillium oxalicum, a biocontrol agent against fungal diseases and against the potato cyst nematodes (PCNs), Globodera pallida and Globodera rostochiensis.We tested the effect of P. oxalicum on the nematode cysts under laboratory conditions or in soil microcosms. A reduction in the rate of G. pallida juveniles hatching by P. oxalicum was observed when root diffusates from the 'Monalisa' and the 'Désirée' potato cultivar were used (98·6 and 74·1% reduction, respectively). However, the rate of G. pallida juveniles hatching was not significantly reduced when root diffusates from the 'San Pedro' tomato cultivar were used. Penicillium oxalicum also significantly reduced the ability of the G. rostochiensis juveniles to hatch (30·9% reduction) when root diffusates of the 'Désirée' potato cultivars were used. Penicillium oxalicum treatment of the soil significantly reduced the number of G. pallida cysts that were recovered from the soil of each pot that contained the 'Désirée' potato cultivar.Our results show that P. oxalicum is a potential biocontrol inoculant for protecting potato crops against PCNs.Penicillium oxalicum has potential to be used in order to reduce PCNs.