The evaluation of environmental effects generated by agricultural production on water quality became essential in Brazil after the creation of policies for the use and conservation of water resources. For such, water quality indices have been considered with the purpose of showing the spatial and temporal variation of water quality in a watershed. The objective of this study was to develop a water quality index (WQI) applying the Multivariate Factorial Analysis (MFA) statistical technique, which could indicate the influence of agricultural activities in the quality of water resources. Water in a predominantly farm watershed was monitored from Sept. 2003 to Sept. 2004. Monthly water collections were carried out at six sample points, and eight parameters were analyzed: nitrate, ammoniacal nitrogen, ammonia, total phosphorus, electrical conductivity, pH, suspended solids and turbidity, which were considered important due to the agricultural management adopted in the region. Results indicated a contamination of agricultural origin along the basin. Factorial analysis showed that ammonia, ammoniacal nitrogen and nitrate parameters were the ones that most contributed in determining the WQI.