Abstract: The survival of Pseudomonas fluorescens Ag1 in seawater of Roskilde Fjord (Denmark) was evaluated by a series of laboratory microcosm and field-based mesocosm experiments. In sterile seawater microcosms, culturability of Ag1 was negatively influenced by high salinity (34 versus 8.5‰). In microcosms with 0.2 µm-filtered seawater, addition of a carbon + nitrogen + phosphorus nutrient mixture was needed to induce proliferation of Ag1. In nutrient-amended microcosms the Ag1 population maintained viability, as determined by the direct viable counts method, at a level close to 100%. In natural water microcosms, Ag1 decreased by two to three orders of magnitude in three days. Field experiments in 5300-litre seawater enclosures demonstrated a less pronounced decline. The observed average decline rate agreed well with the calculated average predation potential of heterotrophic nanoflagellates using fluorescence-labelled Ag1 as prey. During the experiment, mesocosms were amended with nutrients as in microcosms to attempt induction of Ag1 cell proliferation in situ, but the decrease rate of Ag1 remained unchanged. Viability remained above 25% throughout the experiment, emphasizing that the decline of Ag1 did not result from extensive cell death. The combination of micro and mesocosms proved useful as a test scenario for fate studies of microorganisms introduced into an aquatic environment.