The sensitivity of 26 isolates of Bremia lactucae to the fungicide oxathiapiprolin was determined using an in vivo assay. Oxathiapiprolin (FRAC code 49) is the active ingredient in Orondis^® fungicide, which provides exceptional control of diseases caused by downy mildews and Phytophthora species. It is active against mycelial growth, sporangial germination, and zoospore release and germination. Oxathiapiprolin inhibits the oxysterol binding protein, a member of the OSBP related protein family of lipid transfer proteins. Lipid-binding proteins are implicated in many cellular processes related with oxysterol, including signaling, vesicular trafficking, lipid metabolism, and non-vesicular sterol transport. The risk of resistance development is considered to be medium to high and comprehensive resistance management should be implemented when used in the field. The baseline sensitivity for Bremia lactucae was established using 26 isolates never exposed to oxathiapiprolin. Isolates were obtained from lettuce infected samples collected in California, Florida and Arizona. Sensitivity tests were conducted on 3-4 week-old lettuce plants sprayed with oxathiapiprolin and an untreated control. Disease severity was quantified for oxathiapiprolin at concentrations of: 0, 0.00001, 0.0001, 0.001, 0.01, 0.1 and 1 mg.ai/L. Baseline sensitivity distributions (ED50 values) of B. lactucae isolates ranged from 0.000115 to 0.003318 mg.ai/L with a geometric mean of 0.000758 mg.ai/L and a range of 29X. Baseline results can be used as part of a fungicide resistance monitoring program for B. lactucae.