The in vitro antimicrobial activity of the three most commonly used fluoroquinolones (norfloxacin, ofloxacin and ciprofloxacin) was measured for 2235 isolates of seven common pathogenic gram-negative bacteria. Collected from a major teaching hospital in Taiwan during the years 1985-86, 1989-90, and 1996-97, the samples were evaluated using the agar dilution method. The overall susceptibility to fluoroquinolones has decreased rapidly after the wide use of these antimicrobial agents. Isolates of Escherichia coli and Morganella morganii proved quite susceptible to the fluoroquinolones until 1996. By 1996-97, 20% of the bacteria had become resistant to the fluoroquinolones. Activity against Klebsiella pneumoniae and Proteus mirabilis remained excellent, however, with more than 90% of the isolates susceptible during the survey period. Interestingly, some strains of Serratia marcescens and Pseudomonas aeruginosa were demonstrating in vitro resistance even before the fluoroquinolones were launched in Taiwan. This resistance increased rapidly, with around 20-30% of S. marcescens isolates fluoroquinolone-resistant by 1996-1997. After the introduction of the fluoroquinolones, resistance was demonstrated for about 15% of the P. aeruginosa isolates, and 20% of Enterobacter cloacae isolates, with the susceptibility rate remaining steady state through the 1990s. Different pulsed-field gel electrophoresis patterns have been demonstrated for most fluoroquinolone-resistant isolates, suggesting that the increased resistance was not due to the spread of a single clone. We conclude a trend of increasing fluoroquinolone resistance for many common gram-negative bacteria in Taiwan, especially in recent years. Appropriate use of the new fluoroquinolones should be encouraged in order to prevent the rapid emergence and increase of fluoroquinolone-resistant bacteria.