The impact of unionization on male-female earnings differences in Canada is analyzed using data spanning 1981 to 1988, a period in which the male-female unionization gap narrowed considerably. Gender differences in union density, union wages, and nonunion wages are decomposed into characteristics-related and discriminatory components. We find that the drop in the gender unionization gap prevented an increase of 7 percent in the overall wage differential between men and women. Also, male-female earnings differences in the nonunion sector make a substantially larger contribution to the gender earnings gap than do those in the union sector.