Population studies of Dermatocarpon miniatum (L.) Mann. from rock outcrops at Kankakee River State Park, Kankakee and Will counties, Illinois, show that this species is polymorphic for cyanide production. Of the nine populations examined, all individuals from six were 100% strongly cyanogenic while in the remaining populations 67% to 97% of the individuals were cyanogenic. Specimens of D. fluviatile (G. Web.) Th. Fr. were occasionally found to be weakly cyanogenic.