Technical Abstract: Downy mildew, caused by Pseudoperonospora humuli is an important disease of hop in the U.S. Insensitivity to phenylamide fungicides was detected in isolates of the pathogen in production regions in Oregon and Idaho in 1992, and has since been used on a very limited basis. The prevalence of phenylamide insensitivity among isolates of P. humuli collected from 2005 to 2007 in the Northwestern U.S. was quantified using inoculum derived from 201 diseased shoots (basal spikes) and 42 monosporic isolates. Insensitivity to mefenoxam was detected in 31 of 74 (41.9%) basal spikes collected from 13 hop yards. Insensitivity to the related compound metalaxyl was detected in 52 of 80 (65%) spikes collected from 9 hop yards, including four hop yards in Washington State. Log ED50 values ranged from -2.25 to 2.67 for basal spike isolates and -2.27 to 2.98 for monosporic isolates and had a similar distribution. Log ED50 values for monosporic isolates and entire basal spike isolates were significantly associated. However, the slope of the regression line was <1, indicating that the log ED50 values obtained from entire basal spike isolates were greater than the corresponding log ED50 values obtained from a monosporic isolate obtained from that spike. This research suggests that insensitivity to phenylamide fungicides is a stable phenotype in P. humuli. Management of downy mildew should rely on measures other than these fungicides in yards and/or regions where phenylamide insensitivity is prevalent, which is now known to include certain hop yards in Idaho, Oregon, and Washington State.