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Two simply inherited characters, o-hydroxycinnamic acid (o-HCA) content and β-glucosidase activity, were used as genetic marker s in studying the frequency of cross-fertilization in Melilotus alba Desr. Based on the o-HCA marker, the effective cross-fertilization averaged 58% for two harvest years in a biennial population and 67% in a 1-year study with an annual population. Use of the β-glucosidase marker provided a cross-pollination estimate of 72% in the biennial population. The extent of natural crossing in sweetclover appears to be sufficient to restore normal vigor in the production of varieties synthesized from selected inbred lines.