A case of non-nosocomial, spontaneously occurring endocarditis caused by growth of Stomatococcus mucilaginosus on a prolapsed mitral valve is reported. Despite the organism's high susceptibility in vitro the patient responded slowly to antibiotic treatment. Colony adherence to agar surface and absent or weak catalase reaction differentiated this gram-positive coccus from coagulase-negative staphylococci and micrococci.