The "EDB" (from "edible") gene cluster, a variant of the ebo cluster of genes found in many bacteria and algae, allows Pseudomonas fluorescens NZI7 (referred to here as "NZI7") to repel grazing by the nematode Caenorhabditis elegans. The mechanism underlying this phenotype is unknown. Here we report that the EDB cluster is involved in the conversion of tryptophan to (1H-indol-3-yl)-oxoacetamide, indole 3-aldehyde, and other indole-derived compounds. Inactivation of the EDB genes in NZI7 resulted in mutants that lack the ability to excrete indole-derived compounds as well as the ability to repel C. elegans. Heterologous expression of the NZI7 EDB cluster in E. coli cultivated in minimal M9 medium containing 2 mM l-tryptophan also released indole derivatives including tryptophol, 3-(hydroxyacetyl)indole, colletotryptin E, and two new dimeric indoles. Expression of the NZI7 EDB cluster in E. coli, cultured in minimal M9 medium and lacking tryptophan, did not produce detectable levels of indole derivatives. Both (1H-indol-3-yl)-oxoacetamide and indole 3-aldehyde showed repellent activity against C. elegans, revealing the mechanism underlying the ability of P. fluorescens NZI7 to repel grazing by C. elegans.