AbstractRapeseed oil is marked by, 1 too low content of linoleic acid and a too high, content of linolenic acid to meet the nutritional quality requirements. First mutants with a considerably improved poly en OIL fatty acid (t.a.) pattern were low in vigour and seed yield. The present experiments were set out to analyze their mode of inheritance and to check the hypothesis chat a low linolenic acid content of the thylakoid membranes inhibits sufficiently high yields of such mutants for physiological reasons.Six independently induced mutants transmitted the high linoleic and low (or in one cast high linoleic acid content of their seed oil by one recessive gene: two expressed allele mutated allele in an intermediate fashion. Of six double mutants of various origin, three revealed an independent, additive action of two loci for the changed polyenoic f.a. contents, while in the three others the mutant alleles exerted partial dominance over their plus alleles.One or two backcrosses with productive varieties improved the performance of the mutants stepwise over the original types. It is therefore, concluded that mutations towards the desired polyenoic f.a. convent are not necessarily incompatible with high seed yields, when the responsible genes are transferred into an appropriate genetic background. But strong effects of environment on yield, oil content and polyenoic f.a. content of the mutants were still seen in a yield test of second backcross progenies.