Rapeseed ranks third in world oil production. An important breeding objective to improve oil quality in this crop is to lower linolenic acid concentration in the seeds. Previous reports indicate that the concentration of this acid in Brassica napus is determined by two or three nuclear genes. Using DNA-based markers, we have successfully mapped a gene determining linolenic acid concentration in an F2 population derived from crossing the cultivar 'Duplo' and alow linolenic acid line, 3637-1. Linolenic acid concentration in this population ranged from 2.1% to 10.5% with-amean of 6.2%. A RAPD marker, K01-1100, displayed significantly different frequencies between two subpopulations consisting of either high or low linolenic acid concentration individuals sampled from the two extremes of the F2 distribution. Marker K01-1100 segregated in a codominant fashion when used as an RFLP probe on DNA from individuals of this F2 population. The linolenic acid concentration means for the three resulting RFLP genotypes in the F2 population were 4.8% (homozygous 3637-1 allele), 6.4% (heterozygous), and 7.5% (homozygous 'Duplo' allele), respectively. It is estimated that this marker accounts for 26.5% of the genetic variation of linolenic acid concentration in this population.