A new class of agricultural fungicides derived from strobilurins act as respiration inhibitors by binding to mitochondrial cytochrome b. The e †ects of the strobilurin, kresoxim-methyl, on conidia germination, mycelial growth and the protection of apple leaves from scab development were investigated for two isolates of V enturia inaequalis randomly selected from a culture collection. Inhibition of mycelial growth required relatively high doses of kresoxim-methyl for both isolates. In comparison, germination of conidia was (ED 50 \ 1 lg ml~1) highly sensitive for one of the isolates while the level of (ED 50 \ 0É005 lg ml~1), inhibition achieved for the second isolate was 60-fold less (ED 50 \ 0É3 lg ml~1). As deduced from identical sequences of cytochrome b cDNAs prepared from both isolates, the di †erent responses of germinating conidia to kresoxim-methyl were not caused by di †erences in the sequence of cytochrome b as the target site for strobilurin action. Strong synergistic e †ects of salicylhydroxamic acid on kresoxim-methyl inhibitory potency suggested that the reduced kresoxim-methyl sensitivity observed for germinating conidia was caused by interference of the alternative respiratory pathway with inhibitor action. However, this mechanism of target site circumvention in germinating conidia had no adverse e †ects on the protection of apple leaves from scab infection by kresoxim-methyl.