Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in adults and an important agent in other infections, including meningitis, otitis media, and conjunctivitis, etc. The optochin susceptibility test is the most widely used method to discriminate S. pneumoniae from other streptococci. However, 0.8-1.5% of S. pneumoniae contain optochin resistant population. Recently, we experienced three cases of variants of S. pneumoniae with decreased susceptibility to optochin. Equivocal optochin disk test should be confirmed by bile solubility, agglutination test, or DNA probe test. (Korean J Clin Microbiol 2002;5(1):59-61)