The Serratia marcescens SR50 chromosomal beta-lactamase gene (ampC) was cloned and sequenced. It contains 1128 nucleotides encoding a protein of 355 amino acids preceded by 21 amino acids which probably constitutes the signal peptide. The mature protein has a predicted molecular mass of 38,901 Da. About 40% of the amino acid sequence was identical among AmpC beta-lactamases resided in S. marcescens, Citrobacter freundii OS60, Escherichia coli K12 and Enterobacter cloacae P99. All of these enzymes are highly similar around the active site serine at the position 59 of the mature enzyme.