Aims: An analytical protocol has been developed and applied for the detection of glucuronidase activity in marine waters as a rapid alternative approach to assess the microbiological quality of seawaters. Methods and Results: The fluorogenic substrate 4-methylumbelliferyl-beta-d-glucuronide is cleaved to a fluorescent product, methylumbelliferone, by the enzyme beta-glucuronidase, specificto Escherichia coli and closely related enterobacterial species (Shigella ). The results suggest that this test is related to E. coli numbers, as estimated by immunofluorescence, more significantly than to faecal coliform numbers, obtained from culture media. Conclusions: The determination of the potential rate of glucuronidase activity may be used as a diagnostic tool for the indirect estimation of the presence of E. coli in seawaters. Significance and Impact of the Study: The method may be particularly useful in the early warning of seawater pollution, allowing the screening of coastal areas with different contamination levels in reduced time.