Since a number of studies have shown a strong positive correlation between apothecia production and lichen thallus size, it has been suggested that size can serve as an easy measure of lichen sexual fecundity. The reliability of the relationship between size and apothecia production among different environments has not been studied. We measured apothecia development and thallus area in populations of the lichenized fungi Xanthoparmelia cumberlandia and Xanthoparmelia coloradoënsis. Our data suggest that while size is a significant predictor of apothecia production, the relationship between size and apothecia production is not consistent among various environments, nor does size adequately explain variation in apothecia production.