Spontaneous hybridization between durum wheat (Triticum turgidum durum) and Aegilops ovata is regularly observed in nature. The frequency of spontaneous amphiploidy in sympatric populations was estimated at 10−6 (direct in situ observations and germinated seed collected from A. ovata plants). In nursery conditions some genotype combinations gave frequencies that were much higher at 10−3. Genomic in situ hybridization revealed that fertile amphiploids had arisen through unreduced gametes, and that some of them carried wheat − A. ovata recombinant chromosomes. The frequency of production of unreduced gametes is probably genetically inherited. Amphiploids provide a route for gene flow, including that of transgenes, to the wild. Gene flow could potentially be minimized through the choice of wheat cultivars that produce a low frequency of unreduced gametes. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82, 503–510.