Streptococcus agalactiae (Lancefield group B Streptococcus), is the most frequent microorganism involved in neonatal infections through the mother-fetus vertical transmission. It is also responsible for infections in pregnant women, and adults with underlying diseases. The objective of this work was to know the S. agalactiae carrier prevalence in pregnant women who attended to Posadas Hospital, and to study the susceptibility pattern of the isolates to penicillin, erythromycin and clindamycin. From 1203 pregnant women studied, S. agalactiae was recovered in 113, which means a prevalence of 9.39%. Antimicrobial susceptibility was tested to 87 isolates. All of them were susceptible to penicillin, and only 2 isolates were resistant to erythromycin and clindamycin (constitutive MLS mechanism). We emphasize the importance of knowing these last resistance, in the case of beta-lactam antibiotics allergic women.