Streptococcus agalactiae (GBS) causes invasive infections in newborns, being the most frequent the maternal transmission. Epidemiological studies use molecular techniques that assess genetic diversity, including random amplification of polymorphic DNA (RAPD) that is found to be accessible, sensitive and uses arbitrary primers to amplify polymorphic segments of DNA by PCR. The objective was to determine the clonal relationship between GBS strains recovered from mothers and their respective newborns. Four pairs of GBS isolates obtained from vaginal-rectal swabs of mothers and blood cultures of their newborns were studied with RAPD. Primers OPS11, OPB17 and OPB18 were used to select one with the ability to discriminate between non-genetically related strains. The Hunter-Gaston formula that establishes the discrimination index (D) was used; when D>0.90, it is considered that the isolates belong to different clones. The amplification profiles for the eight isolates, using each primer independently, allowed to calculate a D=1 for OPS11, and D=0.84 for OPB17 and OPB18. Therefore, OPS11 was selected for the study of the clonal relationship of the isolates, and similar amplification profiles were found by RAPD for each mother-newborn pair of GBS isolates. Different amplification profiles were observed between pairs of mother-newborn strains, which reveals the discrimination between unrelated strains, confirmed by pulsating field electrophoresis (PFGE). These results indicated vertical transmission for each studied case and robustness of the OPS11 primer. Appropriate conditions of the RAPD trial were found, which is useful for epidemiological studies. Key words: Streptococcus agalactiae, neonatal disease, molecular epidemiology, RAPD technique, vertical transmission