Although the GFP has proven to be a useful tool on its own, combining the use of GFP technology with other luminescent markers or probes is also promising to provide answers to challenging questions in environmental research studied community structures with GFP and tetra- methylrhodamine-labelled probe, GAM42a and simultaneously detected GFP-marked P. putida Green3l and cells belonging to beta and gamma subclasses of Proteobacteria.Community structures were visualized by in situ rRNA hybridization and simultaneous detection of GFP fluorescence.Christensen et al. (1996) used gfp labelled donor cells, and luxAB recipient cells to visually distinguish the two strains in their study on distribution of cells active in conjugation in the transfer of the TOL plasmid. In our laboratory, GFP-marked Pseudomonas sp. UG14Gr was co-inoculated with luxAB-marked Pseudomonas aeruginosa UG2Lr cells in creosote contaminated soil (Errampalli et al., 1998). The two bacteria were detected simultaneously and unambiguously in soil samples for 390 days after inoculation. These studies demonstrate the potential of tracking several introduced microorganisms at the same time in bacterial survival and bioremediation studies.