In boxcosm experiments the impact was studied of organic-matter input and the presence of macrozoobenthos on benthic bacterial abundance and production, oxygen consumption and penetration depth in manipulated marine sediments. Different regimes of organic-matter supply (a single pulse or weekly supplies of Phaeocystis) were applied to boxcosms with and without macrozoobenthos. Both regimes of organic-matter supply affected the top layer of the sediment: an increase in numbers and production of bacteria and consumption of oxygen and a decrease in the oxygen penetration depth (oxic zone). The impact of a single pulse appears to be restricted to a period of about 1 mo. The presence of macrozoobenthos had no clear impact on the bacterial variables in the top sediment layer in either regime of organic-matter supply. In the deeper (anoxic) layers it resulted in an enhancement of the bacterial production in both regimes. Bacterial abundance in these deeper layers also increased in the boxcosms weekly supplied with Phaeocystis. The enhanced bacterial production at deeper layers was not reflected by the benthic oxygen consumption. The presence of macrozoobenthos resulted in a further decrease in the oxygen penetration depth. In the presence of macrozoobenthos a net transport of organic matter from the sediment surface to deeper sediment layers (bioturbation) was assessed, resulting in an increase in total organic-matter content in the sediment and an increase in benthic metabolism.