Design The present study is a randomized clinical trial with a parallel design. After the diagnosis interview, participants will be randomly assigned to two intervention arms: an experimental arm - that will be treated with BA in a group setting in addition to usual care (UC) for fibromyalgia and comorbid depression- and a comparison arm that will receive only UC for fibromyalgia with comorbid depression. The research design is longitudinal and mixed (within and between subjects repeated measures). Outcomes will be assessed in both groups before, during and after the intervention, as well as after a three-month follow-up, by a research assistant who will be blind to group assignment.