Technical Abstract: Thiabendazole (TBZ) is commonly used as a postharvest treatment for control of blue mold in apples caused by Penicillium expansum. Different point mutations in the ß-tubulin gene conferring benzimidazole resistance have been reported in plant pathogens, but molecular mechanisms of TBZ resistance in P. expansum from apple in Washington State are unknown. Determination of TBZ resistance level showed that all 102 TBZ-resistant (TBZ-R) isolates were highly resistant. Sequencing of the majority of the ß-tubulin gene showed that 76 TBZ-R isolates harbored the E198V mutation and 26 harbored the F167Y mutation, and all the sensitive isolates did not possess any of the mutations, indicating that these two point mutations in the ß-tubulin gene were correlated with TBZ resistance in P. expansum from apple in Washington State. There was no association between levels of TBZ resistance and types of point mutations (E198V or F167Y) in the ß-tubulin gene. A multiplex allele-specific PCR assay was developed to detect these two mutations simultaneously. Microsatellite-primed PCR derived presence-absence matrix used to assess the genetic relationship among 56 isolates suggested that the resistance mutations originated several times independently and that there was no correlation between the types of point mutation and the genetic background of the isolates.