The original two strains of Mycoplasma genitalium were isolated from the human urogenital tract. No other strains have been isolated from this site since then. We have recently succeeded in propagating a third strain from a urogenital specimen from a patient with urethritis in Vero cell cultures. By electron microscopy mycoplasmas were demonstrated intracellularly in about 10% of the examined Vero cells. Various stages of penetration into the cells could be observed. The flask-shaped organisms seemed to penetrate into the cells by the tip-end which included a rodlike structure. The intracellular location of normal mycoplasmas were in membrane-bound vacuoles very close to the nucleus, occasionally together with a few disintegrated organisms. In a few cells additional material was entangling the mycoplasmas in the cytoplasmic vacuoles. The potential for intracellular survival of M. genitalium may help the organism to evade the defence mechanisms of the human body. This trait may be considered a pathogenic property which supports the presumption that M. genitalium has clinical importance.