C5+, a mixture of benzene, toluene, xylene, styrene, dicyclopentadiene (DCPD), naphthalene and other compounds, is a byproduct of polyethylene production and has been introduced into the environment via accidental release. The degradation of C5+ was studied using a defined consortium of 11 distinct bacterial strains isolated from C(5+)-contaminated soil. Vigorous growth of individual strains on C5+ was no prediction of dominance in the consortium, when the latter was grown under the same conditions. The defined consortium was able to degrade benzene, toluene, styrene and naphthalene, and to codegrade m-xylene in the presence of toluene or naphthalene. It was unable to degrade DCPD, which was inhibitory when degradation of pairs of C5+ components was examined. The complete C5+ mixture appeared to be the best substrate for the consortium.