Hybrid embryo abortion is an effective crossing barrier in the genus Lens L. It divides the genus into two crossing groups: the Culinaris and the Nigricans groups; but it is also evolving within the Culinaris group. The abortion of the F1 embryo is strongly affected by dominant gene action. In the segregating generations embryo abortion is of a quantitative nature and has both components, dominant and additive. These were expressed differentially in the Lens ervoides × L. culinaris and L. orientalis × L. culinaris cross combinations. The genetic control of embryo abortion was indicated further by the association between marker loci and embryo abortion rates in segregating generations. Embryo abortion was not associated with chromosomal aberrations. The establishment of an embryo abortion reproductive barrier by a single somatic mutation in a locus affecting seed development is proposed as a first step in a build-up of genetic isolation in the genus Lens.