Cossatot leafcup, Polymnia cossatotensis [Asteraceae], is an endemic of the Interior Highlands region of Arkansas. Polymnia cossatotensis was discovered in 1988 and is known only from four sites in Polk and Montgomery Counties of western Arkansas. Because of its extreme rarity, P. cossatotensis is listed as G1 and is considered critically imperiled. We visited Polymnia cossatotensis populations from 8 August to 11 August 2006 where we recorded general site characteristics and associated species, collected soil samples, and established population sizes using line transects. The estimated number of individuals for the species is 33,765 plants of which 33,719 are located in just two of the populations. Our findings emphasize the conservation value of these two largest populations.