A total of 319 bacterial strains isolated from the surfaces of seaweeds and invertebrates were tested for their effects on settlement of Ulva lactuca spores and Hydroides ezoensis larvae in laboratory bioassays. Of the 192 bacterial strains isolated from the surfaces of seaweeds 63 isolates were shown to be inhibitory against the settlement of algal spores and 62 isolates were inhibitory against larval settlement. Thirty-seven percent of the 127 bacterial strains isolated from the surfaces of marine invertebrates were shown to be inhibitory against algal spores and larval settlement. The strain CI4 isolated from an adult Cionaintestinalis showed the strongest inhibitory effect and was identified as Pseudoalteromonas sp. via 16S rRNA gene sequencing. The high proportions of host associated bacteria producing antifouling compounds suggest that these bacteria may help the host organism in the defense against fouling.