As part of a gene tagging strategy to study the developmental regulation of patterns of plant gene expression, a promoterless uidA (gusA) gene, encoding the beta-glucuronidase (GUS) reporter, was introduced into populations of tobacco, Arabidopsis and potato by Agrobacterium-mediated gene transfer. The objective was to generate random functional fusions following integration of the gusA gene downstream of native gene promoters. We describe here a detailed analysis of levels and patterns of gusA activation in diverse organs and cell types in those populations. gusA activation occurred at high frequency in all three species, and unique patterns of fusion gene expression were found in each transgenic line. The frequency of gusA activation was differentially biased in different organs in the three species. Fusion gene activity was identified in a wide range of cell types in all organs studied, and expression patterns were stably transmissible to the T2 and T3 progeny. Developmentally-regulated and environmentally-inducible expression of gusA is described for one transgenic line. Phenotypic variants were detected in the transgenic population. These results demonstrate the potential of T-DNA insertion as a means of creating functional tags of genes expressed in a wide spectrum of cell types, and the value of the approach as a complement to standard T-DNA insertional mutagenesis and transposon tagging for developmental studies is discussed.