The ability of Peridermium pini and Cronartium flaccidum aeciospores and mycelium to infect known (Vincetoxicum spp., Pedicularis spp., Paeonia spp.) and potential (Melampyrum spp., Pyrola sp., Dactylorhiza sp., Solidago sp., Salix sp., Geranium sp. and Maianthemum sp.) alternate hosts was tested. None of the mycelial cultures and only 10% of the aeciospore samples produced uredinia or telia on the tested species suggesting that most aeciospores in Finland belong to the autoecious P. pini. Aeciospores from three locations in northern Finland, however, produced uredinia or telia on Vincetoxicum mongolicum, V. nigrum, V. fuscatum, Paeonia anomala, three P. officinalis cultivars, Melampyrum sylvaticum and Pedicularis palustris either in vitro or in vivo, indicating that these aeciospores belong to the heteroecious C. flaccidum, which occurs sporadically in Finland. Interestingly, the host-specificity of C. flaccidum encountered in Finland was very low (e.g. one sample produced uredinia or telia on eight species). This, added to the wide distribution of Melampyrum spp. over northern Fennoscandia, suggests that the main alternate hosts for C. flaccidum in Finland may be in Melampyrum rather than Pedicularis.