Differentiation of SP from other viridans streptococci (VS) depends on demonstrating optochin susceptibility or bile solubility. However, most laboratories only test for optochin susceptibility. Sporadic cases of ORSP have been reported from Europe but not from the USA. We recently isolated an ORSP from the blood of a 5-week-old female. This strain had no inhibitory zone around the optochin disk but was α-hemolytic, had typical morphology, was bile soluble and produced capsular polysaccharide type 9V. Utilizing PCR amplification of a gene previously shown to be responsible for optochin resistance, DNA sequence analysis of the ORSP and four susceptible strains revealed two distinct amino acid substitutions in the resistant strain. However, differences in the location of the mutations in our strain indicate that it is of different origin from similarly characterized strains isolated in Spain. The role of these mutations in optochin resistance is currently being examined by site-directed mutagenesis. We recommend that invasiveα-hemolytic streptococcal isolates resistant to optochin but exhibiting pneumococcal colonial morphology be tested for bile solubility before being identified as VS.