Biofilms are multicellular communities of bacteria attached to a surface and embedded in a protective matrix. In many cases, the signals that induce biofilm formation are unknown. Here, we report that biofilm formation by the marine bacterium Vibrio fischeri can be induced by the addition of arabinose to LBS (Luria-Bertani-salt), a tryptone-based medium. Growth of cells in the presence of 0.2% arabinose, but not other sugars, induced the production of a pellicle at the air/liquid interfaces of static cultures. V. fischeri failed to grow on arabinose as the sole carbon source, suggesting that pellicle production did not occur as a result of increased growth, but experiments using the acid/base indicator phenol red suggested that V. fischeri may partially metabolize arabinose. Pellicle production was independent of the syp polysaccharide locus but was altered upon disruption of the bcs cellulose locus. Through a screen for mutants defective for pellicle production, we found that loss of motility disrupted the formation of the arabinose-induced pellicle. Among the ϳ20 mutants that retained motility were strains with insertions in a putative msh pilus locus and a strain with a defect in yidK, which is involved in galactose catabolism. Mutants with the msh gene disrupted grew poorly in the presence of arabinose, while the yidK mutant appeared to be "blind" to the presence of arabinose. Finally, arabinose impaired symbiotic colonization by V. fischeri. This work thus identifies a novel signal and new pathways involved in control of biofilm formation by V. fischeri.