The effect of the bacterial strain CI4 on the germination of spores from the green alga Ulva pertusa was assayed and it was found that the bacterial biofilm and cell-free supernatant strongly inhibited spore germination. In attempts to define the chemical nature of the antifouling substance in the supernatant of CI4, the culture supernatants were tested for activity after heat treatment, enzymatic treatments, size fractionation, and separation into aqueous and organic fractions. Results suggest that this bacterium produces an extracellular component with specific activity toward algal spores that was heat-sensitive and between 3 and 10 kDa in molecular size. The exposure of the organic phase fraction to spores showed inhibitive effect on spore germination. Pronase and carboxypeptidase y did not significantly affect the activity of inhibitory component, suggesting that the component was not a protein or a peptide. The bacterium CI4 was identified as Pseudoalteromonas haloplanktis based on the phenotypic characters and 16S rRNA gene analysis.