A fluorimetric assay method for the analysis of beta-glucuronidase (GUS) reporter gene expression in genetically modified plants is described. Optimization of this method for woody plants and a statistical approach suitable for comparisons of gene expression in different transformants or tissues of the same plant is described. Example data from elm (Ulmus procera ) SR4 regenerant plants, shown to be genetically modified by PCR and DNA-DNA hybridizations, in which higher GUS expression levels are found in stems than in leaves demonstrates the utility of this approach.