This study was conducted to estimate the rate of decreased susceptibility to penicillin (MIC > 0.06-1 microg/ml) in Neisseria meningitidis isolates in Istanbul, Turkey. A total of 30 isolates collected during a 1-y period from patients with meningitis and from nasopharyngeal carriers were tested for penicillin and cefotaxime susceptibility using the E-test. Two out of 12 (17%) clinical isolates and 11/18 (61%) nasopharyngeal isolates showed decreased susceptibility to penicillin with MICs in the range 0.094-1.0 microg/ml, giving an overall resistance of 43% (n = 13). These data show that continued surveillance of trends in antimicrobial susceptibility of N. meningitidis is important for detecting the emergence of N. meningitidis strains with MICs > 1 microg/ml which may pose serious therapeutic problems.