One hundred sixteen isolates of Serratia marcescens collected in Showa University Fujigaoka Hospital between April in 1994 and March in 1997 were investigated by O-serotyping, biotyping and antimicrobial susceptibility testing. The results were as follows. 1. Of the total isolates, 37.1 and 24.1% were O2 and O14, respectively, and these values were higher than that of any other serotype. 2. In the hospital, the O2 strains were often isolates in the wards of neurology, plastic surgery, general surgery and ophthalmology, while the O14 strains were often isolated in the wards of urology and orthopedic surgery. 3. The isolation percentages of the biotypes 5307721 and 70405356 with PII 20E and Microscan systems were 81.1 and 50%, respectively. Both biotypes showed typical S. marcescens. There was no relation between O2 or O14 and biotypes. 4. All of the O2 isolates were susceptible to third generation cephems, cefotaxime, ceftazidime and cefpirome, and at least 88% were susceptible to aminoglycosides, whereas the O14 isolates were much more resistant to these antibiotics than the O2 isolates. 5. The isolation percentages of O2 and O14 from urine were 57.1 and 16.3%, whereas those from sputum and pharynx swab were 7.1 and 53.5%, respectively. 6. The isolation percentage of O14 susceptible to gentamicin was very high (96.5%), compared with that of between April in 1991 and March in 1994 (23.3%). Furthermore, increased isolation percentages of the O14 isolates susceptible to gentamicin, tobramycin and amikacin in this period were linked with the decrease in the annual purchased amount of each aminoglycoside and with the decreased isolation percentage from urine. These findings revealed the environments in which the O2 and O14 isolates in this period were predominant over other O-serotypes, while S. marcescens mediated by patients inhabits in the hospital.