Epilithic diatom assemblages were used to evaluate water quality in the Akçay Stream which is one of the essential tributaries of the Büyük Menderes River in the west of Turkey. This branch was polluted especially by agricultural and urban wastes. In this study, the chemical characteristics and epilithic diatoms were investigated in Akçay between June 2001 and September 2002. As a result, 78 diatom taxa belonging to 23 genera were found in total collected from six stations along the stream. Also, the water quality of Akçay was determined according to diatom taxa by using OMNIDIA Software Program (Lecointe et al. 1993). Results show that SLA, DESCY, CIDP, LMA, WAT, CEE and IDAP indices have quite high correlation with the chemical variables and also, DESCY seems to be the best for our study.