The computer program described in this report, MODPATH-LGR, is designed to allow simulation of particle tracking in locally refined grids. The locally refined grids are simulated by using MODFLOW-LGR, which is based on MODFLOW-2005, the three-dimensional groundwater-flow model published by the U.S. Geological Survey. The documentation includes brief descriptions of the methods used and detailed descriptions of the required input files and how the output files are typically used. The code for this model is available for downloading from the World Wide Web from a U.S. Geological Survey software repository. The repository is accessible from the U.S. Geological Survey Water Resources Information Web page at http://water.usgs.gov/software/ground_water.html. The performance of the MODPATH-LGR program has been tested in a variety of applications. Future applications, however, might reveal errors that were not detected in the test simulations. Users are requested to notify the U.S. Geological Survey of any errors found in this document or the computer program by using the email address available on the Web site. Updates might occasionally be made to this document and to the MODPATH-LGR program, and users should check the Web site periodically.