An analysis of the patterns of chiasma distribution within bivalents, in a number of inbred lines of Lolium perenne, reveals a striking potential for variation in this character, as well as a polygenic genetic basis for its control. In parent plants the chiasmata have a regular pattern of localisation which restricts their formation to the ends of the chromosome arms: this strict localisation is progressively lost with inbreeding and novel patterns of distribution are found where chiasmata occur in more interstitial and proximal regions. The variation is continuous and originates from genotypic differences between the different parent plants, rather than from segregation among families deriving from individual parents.