SUMMARYEthiopian mustard (Brassica carinata) genotypes with different contents of oleic acid (C18:1) in the seed oil could be useful for food and industrial applications. The objectives of the present research were to study the inheritance of high C18:1 in the seed oil of different lines of Ethiopian mustard and its relationship with erucic acid content (C22:1). The low C18:1/high C22:1 mutant line L-1806, the high C18:1/high C22:1 mutant line L-482, the high C18:1/low C22:1 mutant line L-2890 and the low C18:1/very high C22:1 mutant line L-1630 were isolated after a chemical mutagen treatment of C-101 seeds (about 94 g C18:1/kg and 450 g C22:1/kg). The high C18:1/zero C22:1 line L-25X-1 was obtained by interspecific crosses of Ethiopian mustard with rapeseed and Indian mustard. Plants of lines L-2890×C-101, L-482×L-2890, L-1630×L-25X-1, L-1630×L-2890 and L-482×L-1806 were reciprocally crossed and F2 and the BC1F1 generations were obtained. Cytoplasmic effects were not observed in any of the crosses. The segregation pattern in F2 and BC1F1 populations differed in the crosses studied. The inheritance of C18:1 content in crosses segregating for this fatty acid was that expected for one (crosses between L-482×L-1806), two (L-2890×C-101) or three (L-1630×L-2890, L-1630×L-25X-1 and L-482×2890) loci. Oleic acid segregation indicated control of accumulation by two segregating genetic systems, one acting on chain elongation from C18:1 to C22:1 and the other involving desaturation from C18:1 to linoleic acid (C18:2). Accumulations of C18:1 and C22:1 were influenced by the same loci (M1, M2, E1 and E2), which control the chain elongation steps leading from C18:1 to C22:1. In addition, C18:1 was influenced by one additional locus (tentatively named OL) involved in control of desaturation of C18:1 to form C18:2. The genetic constitution of the parent lines would be OlOlE1E1E2E2m1m1m2m2 for L-2890, OlOlE1E1E2E2M1M1M2M2 for C-101, ololE1E1E2E2M1M1M2M2 for L-1630, OlOle1e1e2e2M1M1M2M2 for L-25X-1, ol1ol1E1E1E2E2M1M1M2M2 for L-482 and Ol1Ol1E1E1E2E2M1M1M2M2 for L-1806. Transgressive recombinants were obtained from the cross L-1630×L-25X-1, with about three-fold increase of the C18:1 content of the parents (>643 g/kg) and free of C22:1 content, which represent a high potential for commercial exploitation.