Extensive fieldwork in the Mid-Atlantic Coastal Plain led to the discovery of many new populations that seemed referable to Trichothelium angustisporum, a crustose lichen originally described from South America and the United States. Review of that taxon revealed North American populations are uniformly isidiate and represent a distinct, evidently allopatric species which is described as Trichothelium americanum. The circumscription of T. angustisporum is discussed; the types are shown to be lost, necessitating selection of a lectotype, and the lectotype is illustrated to provide comparison with T. americanum.