In a [sugar beet] field comparison of a benomyl-sensitive and a benomyl-resistant strain of C. beticola, the relative proportion of the resistant strain increased after applications of benomyl, benomyl in alternation with triphenyltin hydroxide, or a tank mixture of the 2 fungicides. The increase in the proportion of the resistant strain was followed by a pronounced reduction of effectiveness of benomyl for leaf spot control. Triphenyltin was equally effective for control of both strains. In the unsprayed control the benomyl-resistant strain appeared more adaptable than the particular sensitive strain with which it was compared. Extensive sampling of leaf spot for 3 consecutive years after the cessation of benomyl usage for control of C. beticola in Greece showed that the frequency with which benomyl-resistant strains were isolated tended to remain constant for a given area, including fields that were sprayed with triphenyltin fungicides. Thus, the benomyl-sensitive and benomyl-resistant portions of natural populations apparently are equal in fitness for survival under selection pressures imposed by various field conditions.