ABSTRACT: The catechol 2,3-dioxygenase (C23O) gene, xylE, from the TOL plasmid pathway was used to probe naturally occurring bacterial communities in an intertidal microbial mat. Bound probe was quantified by densitometric analysis of slot blots using colorimetric detection of hybridization between the probe and total DNA extracts from the sediments. The C23O gene encodes the key ring-breaking enzyme of the toluene degradation pathway, of which benzoate is an intermediate. Radiotracer experiments using ¹⁴C-benzoate detected benzoate mineralization in these sediments, corroborating the presence of both the genetic potential and in situ activity for this transformation.