Aim: Leucine aminopeptidase (LAP), an enzyme involved in the decomposition of-natural peptides, was measured in: different marine and brackish ecosystems, together with some environmental and microbiological parameters. Methods and Results: The fluorogenic compound L-leucine-7-amido-4-methyl coumarin was specifically used for the determination of this in situ activity. The enzyme data obtained from this comparative study highlighted the strong spatial and temporal variability of the distribution of LAP in aquatic ecosystems, which was sometimes related to the course of environmental variables such as salinity and organic carbon content. Conclusions: LAP assay has proved to be a rapid method providing useful information on the microbial metabolic processes involved in the mineralization of organic matter. Significance and Impact of the Study: The determination of the potential rates of extracellular enzyme activity is of great ecological importance to extend knowledge on the role played by bacteria in aquatic biogeochemical cycles.