To evaluate antimicrobial activities of tosufloxacin (TFLX), minimum inhibitory concentrations (MICs) of TFLX and other drugs were determined against clinical isolates that were obtained in our laboratory from February of 1993 to January of 1994 (Period I), and from February of 1995 to January of 1996 (Period II). The results are summarized as follows: 1. Compared to MIC90's reported by others for TFLX in the 1980's, those obtained by us against beta-streptococci, Streptococcus pneumoniae, Peptostreptococcus spp., Haemophilus influenzae and Salmonella spp. were similar to, but those obtained against other bacteria were higher than those reported values. 2. Detection frequencies of resistant strains to TFLX were compared between Period I and Period II. MIC90's of TFLX against Staphylococcus spp., Morganella morganii, Providencia spp., Pseudomonas aeruginosa, beta-streptococci, S. pneumoniae and Peptostreptococcus spp. were lower in Period II than in Period I. TFLX-insensitive or -resistant H. influenzae were found among strains isolated in either of the Periods. 3. Most of the TFLX-resistant Gram-negative organisms tested were also resistant to other NQ's. 4. TFLX showed strong antimicrobial activities against penicillin-resistant S. pneumoniae.