Sampling 426 plants from 271 populations of Eriophyllum lanatum, a western North American species complex, revealed 195 diploid (x = 8), 61 tetraploid, ten hexaploid, and five octoploid populations. Polyploids were ∼400% more frequent in taxonomic intermediates. One to four supernumerary chromosomes were found in 13% of the populations. Artificial hybridizations were made in 58 of the 65 possible diploid-level combinations involving the ten varieties of E. lanatum and E. confertiflorum var. confertiflorum. Aberrations, mostly failure to pair normally, were observed in diakinesis or M(1) cells of progeny in 23 of 99 crosses. Studies of pollen stainability in cotton blue-lactophenol and other fertility indicators in 886 F(1)'s from 191 crosses involving 81 populations showed that strong (22-40% pollen stainability) to weak (60-76% pollen stainability) barriers to interbreeding existed among diploids of the E. lanatum varieties and among them and E. confertiflorum var. confertiflorum. Pollen stainability was much higher in progenies of tetraploid, hexaploid, and octoploid intra- and interspecific crosses involving E. confertiflorum var. tanacetiflorum, E. jepsonii, and E. latilobum than in diploid ones, supporting the hypothesis that polyploidy has mainly served to stabilize the products of intervarietal and interspecific hybridizations.