The aim of this work was to isolate bacterial symbionts from nudibranchs and subsequently to determine anti-Methicillin resistant Staphylococcus aureus (MRSA), cytotoxicity and anti-HSV-1 activities of bio-compounds. Fifteen species of nudibranchs were collected from Karimunjawa and five species from Bali, respectively. A total of 245 bacteria isolates were obtained. The anti-MRSA activity screening activity indicated 2 isolates of active bacteria. Ethyl acetate extracts from supernatants, indicating secreted compounds, showed an inhibition zone against MRSA at concentrations of 500-1000µg/ml. DNA sequence analysis showed that the strainKJB-07 from Phyllidia coelestis was closely related to Pseudoalteromonas rubra, the strain NP31-01 from Phyllidia varicosa was closely related to Virgibacillus salarius. The extract of P. rubra was cytotoxic to Vero cells at a concentration of 75 µg/ml. The extract of V. salarius presented no cytotoxicity at concentrations of 5-1000 µg/ml. No anti-HSV-1 was observed. This is the first reported study describing research on anti-MRSA, cytotoxicity and anti-HSV-1 activity of bacterial symbionts from the viscera of nudibranch. Compounds produced and secreted byPseudoalteromonas rubra and Virgibacillus salarius, symbionts of Nudibranch, had potential anti-MRSA activity. Extracts from P. rubra showed cytotoxic effects on Vero cells, whereas extracts from V. salarius did not show cytotoxic effects. Three compounds were identified by LC/MS after purification from culture supernatant. PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.27745v1 | CC BY 4.0