The Brown Trout is considered one of the 100 worst invasive species worldwide and is difficult to manually suppress making it a good target for possible use of the YY Male eradication approach, the first step of which entails the feminization of juvenile male fish. We exposed Brown Trout fry at first feeding (26DPH) to a range treatment dosages (10-30mg) and durations (60-120d). At 150 DPH, project personnel and COFRH staff collected lengths, weights and genetic fin clips from 75 fish in each replicate tank and PIT tagged them for subsequent growout. On the same day, 10 randomly selected fish per treatment group received a health exam. Health exams involved rankings for fin erosion, anomalies of the head and gills, as well as general body appearance along with weights, length and liver weights. On 29 Nov 2021 project personnel conducted the main sex reversal trial sampling via necropsy. Data at 366 DPH collected (target n = 60 fish per treatment group) included total length, weight, visual phenotype and intersex observations. Genetic fin clips were also taken along with the same suite of fish health data from 10 randomly selected test fish as described above. Genetic sex of all fish was subsequently assessed by the Eagle Fish Genetics Lab (EFGL) using Brown Trout sex markers developed and field tested against five western U.S. populations. Feminization rates (FEM) for genetic males were calculated for each treatment group. Based on sex marker results, the genetic sex of all trial fish combined (n = 1099) closely approximated 50:50 at 50.4% male. There was a small amount of variation in the percent genotypic males across the 10 treatment groups, ranging from 42.9 to 55.0% male for combined replicates. FEM rates for these genetic males ranged from a low of 63.1% for fish exposed to 10 mgE2 for 90 days to a high of 94.9% for fish treated with 20mg/kg for 120 days. Intersex ratios were lowest for both groups treated for 120 days. Based on Hepatosomatic Index (HI) trends, we saw no evidence of long-term (366dDPH) effect of exposure to E2 on liver weight from the treatment regimens evaluated. In terms of the other health factors examined, in the treated groups, there were generally more downward rankings for the two fin rankings than for the body and head/gills/eye variables. Pectoral fins appeared to be more affected by E2 exposure than other fins examined, particularly at 366DPH; however at the same time (366DPH) there were no reductions in head/gill/body rankings with the exception of the 20mg 120 d group which experienced a slight reduction. Based on feminization of genetic males, our results demonstrate that Brown Trout need longer exposure to E2 than Brook Trout to attain high rates of feminization. The two 120 day treatment periods resulted > 94% feminization of genetic males with < 3.4% intersex. The next best feminization and intersex results were observed in the 20mg 90d trial at 81.8% and 7.3%, respectively. However, ascertaining the best of these protocols for possible development of a YY Male broodstock will ultimately depend on growout results. A total of 285 fish from the various treatment tanks remained alive at the cessation of the sex reversal trial. These tagged fish are being reared in a communal raceway and will be examined in the future to ascertain long-term survival, growth and time to maturity. The best treatment protocol will depend, in large part, upon survival and maturity schedules of fish from the above three groups.