Symbioses intimate relationships between dissimilar organisms are most often considered as two-partner interactions. In nature however plants can simultaneously interact with a number of symbionts such as the mutualistic mycorrhizal fungi and the parasitic angiosperm dodder. We found that successful shoot parasitism by dodder on plants in a field experiment occurred almost exclusively when the plant roots were colonized by mycorrhizal fungi. Under controlled conditions life expectancy of dodder was significantly greater on mycorrhizal plants than on nonmycorrhizal plants. Furthermore colonization of roots by mycorrhizal fungi increased the growth rate of dodder to 3.4 times the rate on nonmycorrhizal plants. The mycorrhizal effect on dodder growth occurred before the haustoria of dodder had succeeded in penetrating the host. These results suggest that colonization by mycorrhizal fungi had systemic effects on their hosts which altered either the nature of prepenetration dodder signals or the levels of nutrients contained in host stem exudates. These findings could be important for understanding plant - parasite interactions.