Aims: Monitoring bacterial kinetics in food is of great importance in food safety. The targeted micro-organism has to be identified accurately, among competitive flora. Using green fluorescent protein (GFP)-transformed strains is a possible answer to such issues. However, quantitative studies require that this transformation does not alter the micro-organism behaviour: parent and transformed organisms were thus compared. Methods and Results: Three Escherichia colt O157:H7 strains were transformed using a GFP-plasmid expressing. Parent and transformed strains were compared according to their genetic characteristics and serotypes. Growth ability was also assessed in constant and fluctuating temperature profiles. Cardinal values of pH, water activity and temperature were computed. No differences were observed between parent and transformed strains for all these experiments. The plasmid was satisfactorily maintained within transformed strains throughout the studies. Growth was eventually monitored in beef meat. Conclusions: Using the GFP marker is of great value, as it allows easier enumeration of E. colt O157:H7 in food in the presence of natural microflora. Using transformed strains is legitimate: their behaviour does not differ from that of their parent strains. Significance and Impact of the Study: GFP transformation appears to be a valuable and reliable tool for challenge testing studies and predictive microbiology.