Bacteria and fungi associated with the marine wood-boring isopods Limnoria lignorum and Sphaeroma serratum, and with their wood burrows and seawater were investigated. Plate counts using nutrient agar and Czapex Dox agar media in natural and in artificial seawater were used. The diversity and number of bacteria associated with the two isopods were much higher than those associated with wood or seawater, whereas the opposite was true for fungi. The hydrolytic activity of the predominant genera on carboxymethylcellulose and on different types of wood was investigated. The magnitude of fungal cellulase activity exceeded that of bacteria. Activities of bacterial extracellulases exceeded those of intracellulases, while the opposite was observed for fungi. Cellulolytic activity of the predominant isolates biodegraded labkh, peach, white oak and mahogany in decreasing order of activity. Evidence provided by the cellulolytic activities, detected in woodcontaining cultures, suggests that the predominant micro-organisms isolated may play an important role in the woodboring process. Specimens used in the present study were collected from Port Fouad at Port Said Harbour, Egypt.