A new mycoplasma, serologically distinct from all other known mycoplasmas, was isolated from urethral specimens from two of thirteen men with non-gonococcal urethritis. Repeatable isolation and propagation was accomplished by use of a special culture medium. The organisms adhered to glass or plastic, erythrocytes, and monkey kidney cells. This property appears to be associated with surface material restricted to the area of a terminal structure of the flask-shaped mycoplasmas. Although the data are insufficient to implicate the new mycoplasmas in human disease, the fact that they are unique, extremely fastidious, and have adherence properties, has stimulated efforts to assess their pathogenicity and possible role in human urogenital disease.