In the wastewater industry, artificial wetlands are used to improve water quality. Biofilms on these plant surfaces are thought to retain most of the active bacterial community that decomposes organic matter and aid in nutrient removal. Wetland design and operation could be enhanced with the in situ measurement of growth and dynamics of the biofilm-bacteria. This paper describes how to directly measure the rate of bacterial growth on the surface of submerged sections of emergent macrophytes, with the radioactively labelled DNA precursor [methyl-3H] thymidine. We found that the isotope was rapidly and efficiently incorporated into the bacteria growing on plant surfaces, without a lag phase. Isotope dilution was avoided by using a specific activity of 2 Ci.mmol−1. Highest growth rates appeared to be associated with the top 10 mm of submerged plant tissue. The method accommodated the natural heterogeneity of biofilms both between plants and along the stem of the same plant. These findings are important for future studies of biofilm dynamics.