In order to assess the quantitative role of heterotrophic bacterial activity in the cycling of primary produced organic matter, measurements of bacterial biomass, growth and mortality rates were carried out in the Prydz Bay area in February - March 1987. These measurements, along with data from other authors collected in the same area allow to reconcile contradictory opinions published in the literature concerning the significance of the microbial loop in the Antarctic ecosystem in comparizon with temperate marine systems : (i) A similar significant part of primary production is utilized by planktonic heterotrophic bacteria. (ii) A much longer delay in the response of bacteria to phytoplankton development exists in Antarctica. These results are discussed in the light of the ecological structure of the Antarctic Fcosystem