The aquatic environment is the habitat of many microorganisms, including Plesiomonas shigelloides and Aeromonas species which are pathogenic to human and animals. In the present investigation, we evaluated the occurrence of these pathogens from marine mammals beached or accidentally captured by fishing net in southeastern (RJ) and southern (RS) coastal Brazilian regions. A total of 198 swabs from 27 specimens of marine mammals, including 11 different species, were collected by DEENSP and GEMARS-CECLIMAR/ UFRGS Institutes and sent to LRNCEB/IOC/FIOCRUZ. The samples were enriched in Alkaline Peptone Water (APW) added with 1% of sodium chloride (NaCl), APW plus 3% NaCl and incubated at 37ºC for 18-24 hours. Following, samples were streaked onto Pseudomonas-Aeromonas Selective Agar Base (GSP Agar) and suspected colonies were biochemically characterized. The results revealed 114 strains, including ten Aeromonas species and P. shigelloides. The main pathogens isolated were A. veronii biogroup veronii (19.3%), A. caviae (12.3%), A. hydrophila (9.6%) and P. shigelloides (7%). The pathogens were isolated in both coastal and offshore marine mammals. These data point the importance of epidemiological surveillance and microbiological monitoring and reinforce the need to implement environmental protection programs, especially related to endangered cetacean species.