It has long been suspected that Grammia oithona may be a form of G. phyllira, but concrete evidence has been lacking. The only obvious difference between G. oithona and G. phyllira is the presence of cream-colored scales outlining the veins of the forewing of G. oithona. A female G. oithona from Hampden County, Massachusetts, U.S.A., produced progeny consisting of 55 phyllira and 51 oithona. The following year a female G. phyllira from the same locality produced 33 phyllira and 40 oithona. Therefore the name oithona represents a wing pattern phenotype, not a species, at least in the population studied. Progeny of both wild females were bred in captivity, each cross consisting of a virgin female bred with a single male, with eight separate crosses producing offspring. The simplest, most parsimonious hypothesis consistent with the data from all eight crosses is that the wing pattern phenotype is inherited as a single autosomal gene with two alleles, a dominant phyllira allele and a recessive oithona allele; dominance may be incomplete in heterozygotes. Assuming G. phyllira and G. oithona to be conspecific across their composite range, the phyllira phenotype occurs with high frequency in most populations along the East Coast and in the Upper Midwest, and with low frequency in most populations to the west and south of this range. G. phyllira is of conservation concern in the northeastern U.S.A., where it has declined substantially during the past 50 to 100 years. The natural history of G. phyllira is typical of Grammia species, but its dependence on grassland and savanna habitat on dry, sandy soils is an important consideration in conservation and management efforts for this species.