We have identified an unusual group of viridans group streptococci that resemble Streptococcus pneumoniae . DNA-DNA homology studies suggested that a subset of these isolates represent a novel species that may be included in the S. oralis- S. mitis group of viridans group streptococci. We suggest that this novel species be termed Streptococcus pseudopneumoniae . A combination of phenotypic and genetic reactions allows its identification. S. pseudopneumoniae strains do not have pneumococcal capsules, are resistant to optochin (inhibition zones, less than 14 mm) when they are incubated under an atmosphere of increased CO2 but are susceptible to optochin (inhibition zones, >14 mm) when they are incubated in ambient atmospheres, are not soluble in bile, and are positive by the GenProbe AccuProbe Pneumococcus test. The bile solubility test is more specific than the optochin test for identification of S. pneumoniae . Genetic tests for pneumolysin ( ply ) and manganese-dependent superoxide dismutase ( sodA ) and identification tests with a commercial probe, AccuProbe Pneumococcus, do not discriminate between the new species and S. pneumoniae .