During the first six months of 1990 and 1994 respectively, 3179 and 2721 non-repetitive strains of Enterobacteriaceae were isolated at the teaching hospital of Clermont-Ferrand. Extended-spectrum beta-lactamases were found in 147 isolates, mainly in Klebsiella pneumoniae (n = 73), Enterobacter aerogenes (n = 37) and Proteus mirabilis (n = 15). TEM-3/CTX-1, SHV-4/CAZ-5 and TEM-24/CAZ-6 were the prevalent enzymes. However, the most frequently involved enzyme in 1990 was TEM-3 (48.6%) whereas in 1994 it was SHV-4 (56.0%). Two salient features emerged from this study: the high frequency of TEM-3 production in P. mirabilis and the production of SHV-4 enzyme, heretofore observed solely in K. pneumoniae, in two other species, E. aerogenes and Escherichia coli.