The methylmercury criterion adopted as a water-quality standard in the State of Idaho is a concentration in fish tissue rather than a concentration in water. A plan for monitoring mercury in fish tissue and water was developed to evaluate whether fish in the Boise River, Idaho, upstream and downstream of wastewater-treatment plant discharges, meet the methylmercury water-quality criterion. Monitoring also will be conducted at sites on the Snake River, upstream and downstream of the confluence with the Boise River, and in Brownlee Reservoir, which lies along the border between Idaho and Oregon. Descriptions of standard procedures for collecting and processing samples and qualityassurance steps are included. This monitoring plan is intended to provide a framework for cooperative methylmercury sampling in the lower Boise River basin.