Chromosomes in excess of the basic complement have been known for 100 years. Nevertheless, in most species the role of supernumerary or B chromosomes remains unknown or speculative. Here, I describe the frequency, distribution, and transmission to progeny of supernumerary chromosomes in two widely distributed western North American polyploid complexes. Meiotic analyses of microsporocytes found one to four supernumerary chromosomes in 14% of the 293 populations examined in Eriophyllum lanatum, and one, two, or six supernumeraries in 15% of the 133 populations examined in E. confertiflorum var. confertiflorum. Most supernumerary chromosomes were in the size range of A chromosomes, indistinguishable from As, and did not pair with them. Artificial hybridizations showed that the supernumerary chromosomes were transmitted by either parent. Populations with supernumerary chromosomes were nonrandomly distributed in both species. Intervarietal or interspecific hybridization may be responsible for some of their nonrandom distribution in E. lanatum. Supernumerary chromosomes may be involved in the dysploid chromosome numbers in Eriophyllum.