Following masculinization of an all female genetic population of rainbow trout larvae by repetitive immersions with 17α-methyltestosterone (17 αMT) we investigate if we could characterize a L/R gene expression asymmetry in the gonads of these treated fish. The 17 αMT immersion treatment resulted in an overall masculinization percentage of 29%. Gene expression of 18 genes reveals that most of the genes investigated exhibit clear left/right (L/R) differences in their gonadal expression profiles. This L/R asymmetry is probably the result of an early asymmetry of the gonad during the sex differentiation period.