The present paper describes a Pollutrion Evaluator Index (PEI) for coastal aquatic ecosystems. It is based on parameters of water and sediment. For calculating the PEI, the average values of the parameters are compared with their standard limit values for unpolluted ecosystems (described by researchers) and percent variation from the limit values is calculated. Accordingly positive or negative points are given for the variation towards healthy or polluted conditions respectively. The basis of calculation of PEI method is flexible and can be suitably modified for the other dquatic ecosystems (or even the terrestrial ecosystems). The calculation is explained using the 1999-2000 data on Thane creek and the advantages and the limitations of the index are discussed.