The objective of this study was to characterize the gene cassettes of class 1 integrons and antimicrobial resistance among CTX-M-3-producing Serratia marcescens isolates from different specimens in southern Taiwan. One hundred and twenty-two isolates (70.5%) of 173 CTX-M-3-producing S. marcescens isolates were positive for class 1 integrons, including 53.3% of blood isolates, 94.1% of urine isolates, and 87.2% of sputum isolates. No class 2 or class 3 integrons were detected in this study. By PCR with primers 5'-CS and 3'-CS for the amplification of gene cassettes regions, amplicons ranging from 0.7 to 3.0 kb in length were found in 108 (88.5%) of the 122 class 1 integron-containing isolates of CTX-M-3-producing S. marcescens isolates. Ten different types by pattern of amplicons for class 1 integrons were obtained. The Type I amplicon (46.3%) harbors two different class 1 integrons containing the gene cassette of aadA2 and aadB-catB3, respectively, and was most prevalent in the gene cassette region-positive S. marcescens isolates, followed by the Type II amplicon, which harbors one class 1 integron containing the gene cassette dfrA12-orfF-aadA2 (28.7%). Most of the S. marcescens isolates (66.7%, 8/12) harboring three different class 1 integrons (Type IV amplicon) were found in blood isolates. Class 1 integrons were conjugally transferred to recipients in 92.0% of S. marcescens harboring two different class 1 integrons containing the gene cassettes aadA2 and aadB-catB3, respectively. The transfer rate of class 1 integron carrying dfrA12-orfF-aadA2 was detected in 77.4% of S. marcescens isolates. The results showed that all those isolates with conjugative transfer of integrons carried their class 1 integrons on the conjugative plasmids.