The emerging antibiotic resistance genes in the aquatic environment have aroused public concern. As beta-lactam is the most widely used group of antibiotics, beta-lactam resistance genes were selected to investigate their distribution and diversity in the activated sludge from 15 geographically different sewage treatment plants (STPs) of China, Singapore, USA, and Canada. Specific PCR and quantitative real-time PCR (q-PCR) were used to investigate the occurrence and abundance of nine beta-lactam resistance genes. Five genes (OXA-1, OXA-2, OXA-10, ampC, and TEM-1) were detected in most of the sludge collected, while three genes (mecA, CTX-M-1, and SME) were not found in any sludge sample. The total abundances of the six detected beta-lactam resistance genes in the 15 STPs varied from 5.34 x 10(1) copies/ng DNA (ampC) to 5.49 x 10(4) copies/ng DNA (OXA-1). Overall, OXA-1 had the highest total concentration, followed by IMP and OXA-10. Noticeably, the abundances of TEM-1 in Chinese STPs were generally higher than those in the STPs of other countries, while the abundances of OXA-2 and IMP in the STPs of North America were much greater than those of East Asia. A total of 78 clones carrying beta-lactam resistance genes were randomly selected from six clone libraries for phylogenetic diversity analysis; the similarity of these cloned genes to known beta-lactam resistance genes with sequence identities ranged from 96% to 100%. Furthermore, OXA-1, ampC, and IMP were found to be more diverse than the other beta-lactam resistance genes.