The effects of interspecific competition between the potato cyst nematodes (PCN), Globodera pallida and G. rostochiensis, under variable population conditions were examined. PCN-susceptible potato cultivars were inoculated with single-and mixed-species populations of PCN. There was a significant difference in species multiplication between single-and mixed-species populations. Globodera rostochiensis multiplication in mixed-species populations was significantly lower than single-species populations. Density-dependent trials showed a significantly greater increase in multiplication of G. pallida at low inoculation densities relative to G. rostochiensis. These results indicate that G. pallida has a negative effect on G. rostochiensis populations and is the more successful species when competing. A staggered inoculation trial showed that the greater competitiveness of G. pallida was not wholly attributed to its later hatch. Globodera pallida outcompeted G. rostochiensis regardless of the timing of inoculation; however, G. pallida multiplication was lowest when plants were simultaneously inoculated with both species.