Eriophyllum lanatum vars. achilleoides and leucophyllum intergrade in southwestern Oregon. Some populations cannot be unequivocally assigned to either variety. Chromosome counts showed polyploid populations predominating where the varieties meet in southwestern Oregon. Pollen fertility, estimated by cotton blue-lactophenol staining, was the main criterion used to assess barriers to interbreeding. Artificial hybridizations between diploids revealed barriers to interbreeding between vars. achilleoides and leucophyllum, and between each of them and a morphologically intermediate population. The barriers to interbreeding are substantially less developed than those present between most of the other eight varieties of the E. lanatum complex. Supernumerary chromosomes are postulated to be adaptive in Eriophyllum hybrid zones.