The U.S. Environmental Protection Agency's Great Lakes National Program Office (GLNPO) and partners instituted the Lake Michigan Mass Balance (LMMB) Study to measure and model the concentrations of representative pollutants within important compartments of the Lake Michigan ecosystem. The goal of the LMMB Study was to develop a sound, scientific base of information to guide future toxic load reduction efforts at the Federal, State, Tribal, and local levels. Objectives of the study were to: (1) estimate pollutant loading rates, (2) establish a baseline to gauge future progress, (3) predict the benefits associated with load reductions, and (4) further understand ecosystem dynamics. The LMMB Study measured the concentrations of polychlorinated biphenyls (PCBs), trans-nonachlor, atrazine, and mercury in the atmosphere, tributaries, lake water, sediments, and food webs of Lake Michigan. This document summarizes the atrazine data collected as part of the LMMB Study, and is one in a series of data reports that documents the project. Future documents will present the results of mass balance modeling.