Chromosome 1A(m) of Triticum monococcum is closely homeologous to T. aestivum chromosome 1A but recombines with it little in the presence of the wheat suppressor of homeologous chromosome pairing, Ph1. In the absence of Ph1, the two chromosomes recombine as if they were completely homologous. Chromosomes having either terminal or interstitial segments of chromosome 1A(m) in 1A were constructed and their recombination with 1A was investigated in the presence of Ph1. No recombination was detected in the homeologous (1A(m)/1A) segments, irrespective of whether terminally or interstitially positioned in a chromosome, whereas the levels of recombination in the juxtaposed homologous (1A/1A) segments was normal or close to normal relative to completely homologous 1A chromosomes. These observations show that Phl does not regulate chromosome pairing by premeiotic chromosome alignment and a mitotic spindle-centromere interaction, as has been suggested, but processes homology along the entire length of chromosomes.