The Green Fluorescent Protein (GFP) from the jellyfish Aequorea victoria has become a versatile reporter for monitoring gene expression and protein localization in a variety of cells and organisms. In this diploma work the development of two GFP based microbial whole-cell biosensors will be described. The potential to use the GFP strains for determining bioavailable octane concentrations by measuring the intensity of the GFP fluorescence signal will be discussed. One biosensor strain was based on Escherichia coli DH5a. This strain was cotransformed with two plasmids. One of these (pGEc74) carried the regulatory gene alkS from Pseudomonas oleovorans, the other (pJAMA 7(GFP)) a transcriptional fusion of the alkB promoter from P. oleovorans with a promoterless gfp gene from A. victoria. The other biosensor strain was based on P. oleovorans itself. In this case the transcriptional fusion of P alkBwith the gfp gene was integrated on the chromosome by using a mini-transposon delivery system. Epifluorescence microscopy in combination with digital image analysis was applied to detect and quantify GFP fluorescence. Standardized assays were applied to measure induction of E. coli DH5a (pGEc74, pJAMA7(GFP)) with octane, a typical inducer of the alk system in solution. Maximal GFP signal intensity was reached after 4 h induction time. Optimum temperature for induction of GFP and of chromophore formation was 25 °C. A linear increase of GFP fluorescence intensity was found for octane concentrations between 0.5 and 10 μM. The individual variability of GFP expression within the bacterial population was also dependent on the applied octane concentration and the incubation time. At lower concentrations and shorter incubation times only a few bacteria accounted for the total observed fluorescence. At higher concentrations and longer incubation times most of the bacteria accounted for the total observed fluorescence. In contrast to the E. coli based sensor, P. oleovoras::alkB-gfp did not show sufficient induction of the GFP system. This is probably due to a fast simultaneous degradation of the octane applied to the cells, which, therefore, is not contributing to the induction of the alkB-gfp fusion. The GFP-E. coli octane biosensor was also applied on surfaces of agar tubes to determine diffusion of octane from a distant source. With pure octane as source, induction of the alkB-gfp fusion was indeed observed. Experiments with diesel and petrol as distant source did not give significant induction.