Two cell lines with 6 and 16 chromosomes were obtained from a predominantly diploid (2n=4) suspension culture ofHaplopappus gracilis(Nutt.) Gray. The 6-chromosome line had two normal homologues of each of the twoH. gracilischromosomes; the remaining chromosome I had a deletion in the short arm, while the remaining chromosome II had a translocated piece of chromatin on the short arm forming a larger satellite than normal. The karyotypic composition of the 4- and the 6-chromosome lines remained fairly stable during thein vitroculture. In the 16-chromosome line, however, a dominant karyotype with 13 chromosomes evolved during thein vitrocultivation. Loss of chromosome(s) by fragmentation appears to be the major method of the production of aneuploid cells. Selection was found to be ‘conservative’ in the 4- and the 6-chromosome lines, while the 16-chromosome showed a ‘progressive’ selection.