Alnus glutinosa (black alder) populations are known to exhibit a variable degree of incompatibility to root nodule formation by ineffective Frankia. The relationship between the occurrence of ineffective Frankia in wet stands of black alder and the degree of resistance to nodulation by ineffective Frankia of seed-lots and clones of alder trees from these particular locations was studied through soil inoculation experiments. The average percentage of resistant plants (R-frequency) among the seed-lots from locations with an ineffective Frankia soil population was equal to, or higher than, the R-frequencies of locations without ineffective Frankia. The mean R-frequency was highest for the seed-lots from the location from which the soil inoculant was taken. These results strongly suggest that ineffective Frankia are not strictly dependent on susceptible A. glutinosa for the maintenance of their population size. The fungus Penicillium nodositatum also nodulated A. glutinosa seedlings. Whereas a negative interaction with the ineffective Frankia nodulation was found, this did not have a significant effect on the R-frequencies of the seed-lots that were tested, suggesting that the ineffective Frankia nodulation adversely affected the myco-nodulation, and not vice versa.