Aims: To determine the source material and the responsible micro-organisms of 4-ethylphenol production in swine lagoon sediment slurries. Methods and Results: Swine lagoon sediment was blended and incubated with tryptone-yeast extract broth containing 10 mmol l-1 each of 4-hydroxycinnamic as well as other phenolics. 4-Ethylphenol was only produced from 4-hydroxycinnamic acid. Denaturing gradient gel electrophoresis analysis indicated that the microbial community was substantially altered from inclusion of 4-hydroxycinnamic acid. Serial dilutions and selective plating were employed to isolate a culture capable of 4-hydroxycinnamic acid conversion to 4-ethylphenol. Morphological and ribosomal gene analysis indicated the isolate to be a Lactobacillus sp., Lactobacillus sp. pep8 converted 4-vinylphenol, but not 4-hydroxy-3-phenylpropionate, to 4-ethylphenol and did not convert 4-hydroxy-3-methoxycinnamic acid to 4-ethyl-2-methoxyphenol. A p-coumaric acid decarboxylase (pdc) gene identical to Lact. plantarum pdc genes was cloned from Lactobacillus sp. pep8. Conclusions: Swine lagoon sediments produce 4-ethylphenol from 4-hydroxycinnamic acid due, in part, to the activity of Lactobacillus spp. Significance and Impact of the Study: 4-Ethylphenol, a malodourant of swine and beef wastes, is generated from plant materials by indigenous lactobacilli suggesting that altering the amount dietary of plant material may influence levels of 4-ethylphenol in the wastes.