A dehydrogenase enzyme assay has been developed to measure microbial activity in petroleum-contaminated beach sediments. The assay, using 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazoliumchloride (INT) as the substrate provides a reliable measurement of overall indigenous microbiological activity in beach sediments containing similar to7% (w/w) oil. The method has been effectively used in field studies (Mathew et al. 1999) to monitor microbial activity in conjunction with the biodegradation of petroleum hydrocarbons.