SUMMARYThe first microconidial dimorphism in noncrustose lichen-forming fungi to have ecogeographical correlations is reported from two North American species. The microconidia ofParmelia hypoleucitesand those of the new speciesP. semansianadiffer in length, the former averaging about twice (11-12μm) as long as the latter (5μm).Parmelia hypoleucites, apparently an obligate epiphyte of trees, is confined to the highlands of Mexico and Arizona.Parmelia semansiana, primarily a rock-inhabiting species that extends its substrate tolerance to bark at the center of its range, is widely distributed in south-central North America. Small but significant spore-size differences accompany the microconidial dimorphism.