peer reviewed ; Background Even if Group B streptococcus (GBS) disease risk is highest during the first 3 months of life, GBS also causes significant morbidity and mortality among adults, especially in the elderly and immunocompromised or with chronic illnesses. We here provide an overview of bacteriological and clinical characteristics of GBS causing invasive diseases in non-pregnant adults in Belgium. Methods Overall 143 GBS strains isolated from invasive diseases among non-pregnant adults sent, on a voluntary-base, to the National Reference Centre (NRC) during the year 2015 by any laboratory located in Belgium were characterized: capsular polysaccharide (CPS)-typing by agglutination and/or with PCR, pili-typing with PCR, antimicrobial susceptibility testing, and detection of resistance genes with PCR. Results Among adult invasive isolates, CPS-type Ia was predominant (25.1%) followed by V, III, II, IV, Ib (23%, 19.6%, 9.8%, 9.8%, 7%) and VI, VII, VIII (1.4% each), IX (0.7%). One strain remained non typeable even with PCR. All strains were susceptible to penicillin. Rate of resistance to macrolides/lincosamides was 35.7%. ErmB, ErmTr and MefA genes were detected respectively within 45.2%, 27.4% and 19.5% among the resistant strains. One strain presenting the L profile (isolated resistance to clindamycin) harboured the LsaC gene. About pili-typing, the combined PI1, PI2a genes were predominant with 51.2% of the cases, followed by PI2a alone, the combined PI1, PI2b and PI2b alone (33.3%, 14.7%, 1.4%). The vast majority of strains were isolated from blood: 43% of bacteremia without reported focus, 19% originating from skin-and-soft tissue infections, 4.9% urosepsis, 4.2% bone-and-join infections, 2.8% endocarditis and 1.4% meningitis. Conclusion Among invasive adult diseases, GBS bacteriological characteristics were consistent with reported data among European countries. Macrolides/lincosamides resistance rate has slightly increased. Bacteremia without identified focus and skin-soft tissue infections were highly ...