In heterozygous females of tandem duplication Dp(1; 1) Gr, y(2) (w(-)spl sn(3)) (w(c) sn(3)) the frequency and distribution of exchange events were measured by the phenotypically different F1 recombinants. In comparison with wild type chromosomes the crossover values were strongly reduced within the heterozygous chromosome sections (between white and singed from 19.5 per cent to 2.1 per cent). Additionally, there was 0.24 per cent intrachromosomal exchange in the same chromosome section after the formation of a double loop (Fig.2). Crossing over was also reduced in both chromosome regions adjacent to the heterozygous duplication. A comparison of the conceivable pairing configurations with those configurations which are indicated by the recombinants gives an initial insight into the pairing behaviour of heterozygous tandem duplications during meiosis. The hypothesis of "polarized pairing" in Drosophila melanogaster is disproved by this kind of experiment.Crossover values within and adjacent to the duplication were also measured in heterozygous tandem duplications differing in the genetic length of their duplicated section. Here, meiotic pairing behaviour was found to depend on the genetic length and other parameters of the duplicated section. Conceivable sources for the different kinds of behaviour are discussed.