A total of 36 species of butterflies was documented from Plummers Island, Maryland, based on four sources: transect survey (2006), diurnal collecting (2005), Malaise trap sampling (2005–2006), and historical records (1904–1997). Each of the four sources contributed species not recorded in the other three, demonstrating the importance of combining methods when compiling inventories. Twenty-five species (69% of the total) were observed during a 10-week transect survey in 2006, with six species not recorded in other sources. Nineteen species (53%) were captured during diurnal collecting in 2005, with five species not recorded in other sources. Eight species (22%) were collected in Malaise traps, with one species not recorded in other sources. Ten species (28%) were represented by historical records in the collection of the National Museum of Natural History, Washington, D.C., with four species not recorded from other sources. The number of species (i.e., 36) is less than what would be expected based on previous inventories of the butterflies of the Washington, D.C. area, and this is most likely the result of the under-sampling.