<jats:p>Exploration of rhizobacteria of the genus <jats:italic>Burkholderia</jats:italic> as an under-tapped resource of bioactive molecules resulted in the isolation of two new antimicrobial 2-alkyl-4-quinolones. (<jats:italic>E</jats:italic>)-2-(Hept-2-en-1-yl)quinolin-4(1<jats:italic>H</jats:italic>)-one (<jats:bold>1</jats:bold>) and (<jats:italic>E</jats:italic>)-2-(non-2-en-1-yl)quinolin-4(1<jats:italic>H</jats:italic>)-one (<jats:bold>3</jats:bold>) were isolated from the culture broth of strain MBAF1239 together with four known alkylquinolones (<jats:bold>2</jats:bold> and <jats:bold>4</jats:bold>–<jats:bold>6</jats:bold>), pyrrolnitrin (<jats:bold>7</jats:bold>), and BN-227 (<jats:bold>8</jats:bold>). The structures of <jats:bold>1</jats:bold> and <jats:bold>3</jats:bold> were unambiguously characterized using NMR spectroscopy and mass spectrometry. Compounds <jats:bold>1</jats:bold>–<jats:bold>8</jats:bold> inhibited the growth of the marine bacterium <jats:italic>Tenacibaculum maritimum</jats:italic>, an etiological agent of skin ulcers in marine fish, offering new opportunities to develop antibacterial drugs for fish farming.