Fields trials were carried out using a 1-hour fluorimetric assay of β-D-galactosidase activity alongside conventional membrane filtration to detect faecal coliforms in beachwater samples. The ultimate aim of the study was to test the reliability of the assay with a view to its use in the field to assess the compliance of coastal bathing waters with the guideline concentrations. The assay had a 99% success rate at detecting pass/fail at 300 faecal coliforms per 100 ml, using a threshold fluorescence of 60.3 nM. A good correlation (r=0.90) between faecal coliform concentration and fluorescence assay results was obtained. The assay provides a rapid, simple and inexpensive method for the detection of sewage pollution in marine waters, and with the aid of portable instrumentation, it may be performed in the field, allowing real time monitoring of water quality.