The (I; II) duplication strain of Aspergillus nidulans contains a terminal segment of chromosome I twice, one copy being in the normal position, and the other one translocated to a tip of chromosome II. We show that in selfed cleistothecia (homozygous for the duplication) crossing-over between the two segments of the duplication followed by the appropriate disjunction and distribution of the meiotic chromosomes, results in segregation of recessive markers in the duplication. Alternative segregation mechanisms like the deletion mechanism responsible for mitotic non-conformity, cannot explain the observations. The average segregation frequency of the marker yA2 is 11%. Using the mathematical model outlined in the Appendix to this paper, we estimated the recombination frequency between the translocation breakpoint and the yA locus from the segregation frequency. The essential feature of the model is the computation of the probabilities of combinations of different tetrad types in quadrivalents. Application of the model leads to the qualitative conclusion that the meiotic recombination frequency within the duplication is increased over the normal level. Elsewhere in the genome the frequencies are either unchanged or decreased. Also in crosses heterozygous for the duplication, an increase by at least a factor of two is found. As judged from the appearance of certain recombinant classes, the quadrivalent frequency in homozygous and heterozygous crosses is 2/3 (random pairing) or slightly higher. The increased recombination is attributed to the same lesions that are responsible for the occurrence of deletions in duplication strains of A. nidulans.