Résumé : Bacterial diversity and activity were simultaneously investigated by microbial ecological and molecular biological methods along an estuarine gradient from the Rhone River to the Mediterranean Sea. Following a Lagrangian strategy, we sampled plume, frontal and marine layers. The sampled estuarine gradient exhibited large changes both in physico-chemical and in microbiological characteristics. Bacterial abundances and activities showed a more drastic decrease in the low salinity range of the gradient than expected from simple dilution models, indicating that an important fraction of freshwater bacteria disappeared in the mixing area. High specific activities, in particular for leucine, in the marine end-part of the gradient suggested important bacterial protein synthesis, which may be a sign of an active survival strategy for bacterial communities subjected to oligotrophic conditions. Bacterial genetic diversity of the sampled estuarine area, as estimated by the number of DNA-derived denaturing gradient gel electrophoresis (DGGE) bands, was high (13 to 55 bands) compared to that reported in other aquatic ecosystems. This high diversity may be the consequence of the interface position of estuaries. The proportion of active populations was estimated using the ratio of DGGE bands derived from RNA and DNA. This ratio was lower in Rhone water than in marine water, indicating that only a part of the constitutive populations were active, while the activity was distributed within a larger fraction of populations in the marine assemblage. Very few DGGE bands detected in freshwater samples were also detected in the marine end-part of the gradient, suggesting that a very limited number of freshwater bacteria could survive under marine conditions. Detection of these freshwater populations from RNA might indicate that these bacteria were able to synthesize different stress proteins as the result of a survival strategy or that these bacteria were able to maintain metabolic activity under marine conditions. The structure of marine communities was strongly affected by decreasing salinity. However, it seems that the decrease of DNA-derived bands may simply have been the consequence of the mixing of marine and freshwater. No obvious relationship between genetic richness and activity changes was observed. This lack of a relationship may be the consequence of a very short residence time of water in the mixing area studied.