A cytological analysis of 1192 artificially produced hybrids in the collective apo-amphimictic species complex, Hieracium pilosella/H. peleterianum, showed that 16 addition hybrids were formed; 15 of these were produced by the fusion of one reduced and one unreduced gamete and one by the fusion of two unreduced gametes. Zygotic chromosome doubling could be unambiguously ruled out. Polyspermaty and somatic doubling in meristematic tissue of the creeping stolons did not occur. The percentage (1-26%) of unreduced gametes is low but sufficient for the production of considerable numbers of addition hybrids, even in moderately large populations. The fact that addition hybrids are rare under natural conditions suggests that the adaptive value of these hybrids is inferior.