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Isozyme analysis of Brassica napus cv 'Topas' and CGRC5006 as well as of Sinapis alba cv 'Emergo' revealed significant polymorphism between the two species for the isozymes, aconitate hydratase, glucose phosphate isomerase, and diaphorase. F1 hybrids between B. napus '5006' and S. alba cv 'Emergo' were backcrossed to B. napus cv 'Topas', and the S1 progeny of the first two backcrosses were studied isozymically. At the backcross one level the frequency of S. alba or S. alba plus B. napus patterns observed ranged from 18% to 87% across the four lines studied. There were differences between lines for the frequency of S. alba patterns, which could have an impact on the efficiency of selection for subsequent backcrossing. By the backcross two generation in one of the two lines studied, GR86-24, the S. alba patterns for GPI and DIA had been lost, while in the other line, GR86-28, the S. alba pattern for ACO had been lost, resulting in lost opportunity for S. alba gene transfer. In a wide cross such as S. alba x B. napus, which requires an intensive effort to accomplish, the isozymes ACO, GPI, and DIA may serve as useful markers to ensure gene transfer between the two species has occurred. In addition, the identification of lines with divergent isozyme patterns from B. napus will provide the basis for establishing linkages between S. alba traits of interest and isozyme markers.