ABSTRACT. – Interactions between benthic macrofauna and bacterial communities in the sediment are very complex. The most conspicuous animal effects on sediment decay processes are caused by burrowing and ventilation activity and by direct feeding on detritus and associated microorganisms. Microbial activity can be stimulated by animals due to fractioning of particles. The direct assimilation of detritus components by macrobenthic organisms can however competitively remove substrates that are otherwise available to microbial decomposers. The aim of this work was to study the possible interactions between macrofauna and bacterial community by a laboratory experiment using microcosms and INT to quantify active bacterial populations with two different temperature values (10^oC and 21^oC). Hediste diversicolor (Polychaeta: Nereididae) – a detritivore polychaete – was considered in this study. This species is known as an important bioturbator of shallow coastal sediments. Results showed that the presence of polychaetes in the sediment (muddy and sandy) reduced bacterial activity at 21^oC while no differences were detected at 10oC. The temperature seemed to play a fundamental role in determining the type of interactions between polychaetes and bacterial community in sediment. The results were similar in both type of sediments.