AbstractThe uptake and metabolism of triadimefon by mycelia of fungi, sensitive or resistant to triadimefon, were studied. In the mycelia the triadimefon content accumulated to 20‐40‐fold of the external concentration, irrespective of the sensitivity of the fungus. In the course of the metabolism a highly fungitoxic product, triadimenol, was formed. In mycelia of sensitive fungi this transformation was at a high rate whereas it could not be demonstrated or only to a low extent in resistant types. The material produced by the sensitive fungi was effective also against fungi basically resistant to triadimefon. Based on these observations, triadimefon has to be regarded as the precursor of triadimenol. In the species investigated, selectivity may be related mainly to the rate of activation. In higher plants the metabolism of triadimefon showed a similar pattern, but was slower than with fungi. As the transformation into triadimenol (activation) begins thus early in the host tissues, the systemic fungitoxic effect is also influenced by the host plant itself. Regarding the speed of activation differences were found between the four plant species investiga