Within F. arundinacea ten exotic populations each crossed with an indigenous bred form S. 170, and their hybrids, were grouped into fertile, partly fertile and sterile categories. Four were fertile hybrids with bivalent pairing (20.75 to 20.99/cell) and high pollen fertility. Two were partly fertile with univalents, and bivalents (18.26 to 19.54/cell) and rarely multivalents. The pollen fertility was low. Four hybrids were completely sterile with low or nil pollen fertility. The bivalent formation was low (12.94 to 14.25/cell) while the frequency of univalents and multivalents was high. There was evidence of structural changes of both gross (multivalents formation due to translocation; univalents due to inversions and deletions) and cryptic types from these hybrids. This diversity in the chromosomal constitution of several populations could be the result of several factors, including wide geographical distribution, climatic and edaphic diversity of the population, polyploid nature of the species, restriction of the gene pool and successful mode of vegetative propagation. In some of the populations an isolating mechanism has been effectively established and may well represent an initial stage in speciation.