The temporal variability of the abundance and the incorporation of (3)H-thymidine and (14)C-glucose by attached and free-living bacteria, as well as their relation with environmental factors, were analyzed in a coastal marine ecosystem during a year. Both communities were quantitatively very different. Attached bacteria represented only 6.8% of the total bacterial abundance, whereas free-living bacteria represented 93.2%. The environmental factors most closely linked to the abundance and activity of free-living bacteria were temperature and the concentration of dissolved nutrients. Moreover, the free-living community showed similar temporal variations in abundance and in activity, with lower values in the cold months (from October to May). The attached community did not present the same pattern of variation as the free-living one. The abundance of the attached bacteria was mainly correlated to the concentration of particulate material, whereas their activity was correlated to temperature. We did not find a significant correlation between the abundance and the activity of the attached community. On the other hand, the activity per cell of the two communities did not present a clear temporal variation. Attached bacteria were more active than free-living ones in the incorporation of radiolabeled substrates on a per cell basis (five times more in the case of glucose incorporation and twice as active in thymidine incorporation). However, both communities showed similar specific growth rates. The results suggest that the two aquatic bacterial communities must not be considered as being independent of each other. There appears to be a dynamic equilibrium between the two communities, regulated by the concentrations of particulate matter and nutrients and by other environmental factors.