Of the two potato glycoalkaloids, α-chaconine and α-solanine, α-chaconine was the more inhibitory on spore germination in Alternaria brassicicola and Phoma medicaginis, and on growth in liquid culture of these species and also Ascobolus crenulatus and Rhizoctonia solani. At 50 μM or less, solanine caused little (<20%) or no inhibition but, in combination with comparable concentrations of chaconine (which were sometimes also below the activity threshold), synergistic inhibitory effects (up to 100% inhibition) were apparent. A scanning electron microscopic examination of agar-grown cultures of P. medicaginis revealed that both glycoalkaloids at 100 μM inhibited hyphal extension, but that chaconine particularly increased branch formation (three-fold) as well as hyphal thickness and ‘beading’. Glycoalkaloid-containing liquid medium from R. solani cultures did not contain glycoalkaloid hydrolysis products. There was a 17% reduction in the solanine content of the medium, but the chaconine level was not significantly different from that in uninoculated controls. Neither glycoalkaloids nor their hydrolysis products were detectable in the washed mycelium of such cultures.