In general, the invention is directed to a medical device implantable in a body of a patient. The device includes a housing with a plurality of collapsible fixation structures coupled to the housing, and can be in a collapsed configuration or an expanded configuration. The device assumes a collapsed configuration when in the bore of an insertion device, and assumes the expanded configuration when expelled from the insertion device into the body of the patient. The extended fixation structures engage the tissues in the body and restrict migration. One exemplary application of the invention is in the context of a microstimulator, with a pulse generator housed in the housing and one or more electrodes coupled to the housing. The fixation structures help keep the electrodes proximate to the tissues that are to receive the stimulation.