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Aims The present study evaluated the antimicrobial activities of the medicinal plant Mallotus japonicus against the fish pathogenic bacteria, Aeromonas hydrophila, Aeromonas salmonicida, Edwardisella tarda and Vibrio anguillarum, and also describes the antimicrobial activities of the major and minor active compounds present within the plant extract. The synergistic effects by way of combination of these compounds were also evaluated and described. Chemical constituents of the plant extracts were analysed using the liquid chromatography‐mass spectrometry (LC‐MS) and described. Methods and Results The diethyl ether‐extract of the plant elicited the strongest antibacterial activity against the challenged bacterial species, followed by ethanol‐ and methanol‐extracts. The major active compound of the extracts, bergenin, demonstrated no antibacterial activity, but other compounds in the extracts did. Conclusion Mallotus japonicus could be used as a prophylaxis to treat bacterial disease infections of fishes and its diethyl ether‐extract has the potential of an alternative to antibiotic treatment in aquaculture. Significance and Impact of the Study Mallotus japonicus diethyl ether‐extract has the potential of an alternative to antibiotic treatment in aquaculture.

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