The common reed, Phragmites australis, has spread throughout Gulf and Atlantic Coast marshes of the U.S. in the past thirty years. In the Chesapeake Bay area, natural resource managers are uncertain as to the current distribution or recent colonization rate of this low-salinity marsh grass. A geographic information system (GIS) was used to determine the distribution and expansion rate of P. australis within four brackish and three tidal freshwater marshes in the upper Chesapeake Bay region. Vegetation patterns were mapped by interpreting aerial photographs from the 1930s, 1970s, 1980s, and 1990s. The aerial extent of P. australis stands was measured by digitizing vegetation boundaries, correcting for distortion, and analyzing the data using a GIS. A geometric growth formula documented an intrinsic rate of increase for each P. australis stand. In addition, aboveground biomass was sampled from monotypic stands of P. australis. Results show that P. australis is present in all seven marshes; however, it is most pervasive in the three tidal freshwater marshes. Based on interpretations of photographs, P. australis was present prior to 1938 in these three marshes. In each successive time period, there was a net increase of P. australis in all seven marshes that were examined. In the past ten years, however, the rate of increase has declined or stabilized in each marsh in which P. australis was well-established prior to 1985. This slowdown is especially prevalent in the three tidal freshwater marshes. In contrast, the three brackish marshes most recently colonized by P. australis showed high annual intrinsic rates of increase (0.06 to 0.19 yr⁻¹ or more). Also, the highest biomass estimates were found in two of these three marshes.