Group B streptococci (GBS) colonizing the vagina and rectum of pregnant women cause invasive disease of the offspring in a small number of cases. The immune status of the host and differences in virulence among strains appear to be the main determinants for neonatal infection. A high-virulence clone (HVC) was proposed to cause much of the morbidity and mortality when a collection of GBS isolates was examined by multilocus enzyme electrophoresis. HVC isolates could be further distinguished by their inability to grow at 40 degrees C. This characteristic was used in the present study to examine a collection of 57 GES isolates from Mexico City for the HVC. Three serotype III invasive strains were classified in the HVC. The other eleven invasive strains and all carrier isolates had growth curves unaffected at 40 degrees C. These results demonstrate the presence of the HVC in Mexico. Such a low prevalence could explain in part the low rate of GBS invasive neonatal disease in Mexico.