Streptococcus (S.) castoreus was isolated from seven beavers (Castor fiber), which were found dead in the federal state of Brandenburg, Germany, and submitted to the Berlin-Brandenburg State laboratory for post mortem diagnostics. The isolates originated from various inflammatory processes where they were associated with other Gram negative and Gram positive aerobic and/or anaerobic bacteria (i. e. Actinobacillus sp., species of the Actinomycetaceae family, coliform bacteria, Fusobacterium sp., Prevotella sp.), but also from cloacal swabs. Testing their antimicrobial susceptibility, all S. castoreus isolates and the type strain S. castoreus (DSM 17536) were classified as being susceptible to penicillin G, ampicillin, amoxicillin-clavulanic acid, 1st generation cephalosporins, erythromycin, tetracycline, and chloramphenicole but intermediate to gentamicin. Considering both the polymicrobial flora isolated from the inflamed tissues and the fact that antimicrobials of a narrow spectrum (namely penicillins, 1st generation cephalosporins, macrolids and lincosamids) can be toxic to rodents, chloramphenicol might be a suitable drug for treatment of beavers suffering from S. castoreus (mixed)-bacterial infections.