The prevalence of clinical bacteria, as isolated from Linko Chang-Gung Memorial Hospital (2,300 beds) in the period January 1985 to December 1986 and from Taipei Veterans General Hospital (2,300 beds) during the period January 1986 to December 1986, was analyzed with the following findings: (i) The isolation ratio of anaerobic and aerobic or facultative bactria during the period of investigation were 7.8% (5,513/70,799) and 92.2% (65,286/70,799), respectively. (ii) Of the total aerobic or facultative isolates from the two hospitals, 32.9% (21,510/65,286) were Gram positive cocci and bacilli, 67.1% (43,776/65,286) were Gram negative cocci and bacilli. (iii) Of these Gram-negative bacilli, 65.2% (28,490/43,675) were Enterobacteriaceae were Enterobacteriaceae and glucose fermentative Gram negative bacilli, 32.0% (13,984/43,675) were glucose nonfermentative Gram negative bacilli, and 2.7% (1,200/43,675) were fastidious Gram negative bacilli. (iv) The more common species among the members of Enterobacteriaceae were Escherichia coli 35.7% (10,163/28,490), and Klebsiella pneumoniae 18.2% (5,186/28,490). The other common species included Enterobacter cloacae, Proteus mirabilis, Serratia marcescens, Morganella morganii, Citrobacter freundii and Proteus vulgaris. The frequencies of Salmonella species and Shigella species in these two large hospitals were up to 1.6% (456/28,490) and 0.5% (149/28,490), respectively. The most common isolate among other glucose fermentative Gram negative bacilli was Aeromonas hydrophila 3.0% (843/28,490). The finding of 0.1% (11/28,490) Vibrio alginolyticus was considered as clinically significant in Taiwan. (v) Of these glucose nonfermentative Gram negative bacilli, 69.4% (9,704/13,984) were Pseudomonas aeruginosa, 18.9% (2,637/13,984) Acinetobacter species, 10.8% (1,516/13,984) Pseudomonas species. (vi) The most common bacteria among fastidious Gram negative bacilli was Haemophilus influenzae, 96.2% (1,154/1,200). (vii) Of these Gram negative cocci, 59.4% (60/101) was Neisseria gonorrhoeae and 6.9% (7/101) N. meningitidis. (viii) The more common isolates of Gram positive bacilli included Bacillus species and Corynebacterium species (diphtheroids). (ix) Of these Gram positive cocci, the isolation rates of Staphylococcus species and Streptococcus species were 54.6% (10,838/19,827) and 45.4% (9,002/19,847), respectively. The most common isolate among Gram positive cocci was Staphylococcus aureus, 30.2% (5,994/19,847); the next, enterococcus, 24.9% (4,936/19,847); then S. epidermidis, 22.2% (4,390/19,847). The less common isolates were Streptococcus pyogenes 1.1% (212/19,847) and S. pneumoniae, 1.7% (329/19,847).(ABSTRACT TRUNCATED AT 400 WORDS)