Homogeneous streamflow regions in Turkey were identified using a multivariate statistical tool; namely, principal components analysis (PCA) with a varimax orthogonal rotation. The rotated PCA was applied to monthly streamflow records of 78 gauging stations during the period 1964-1994. The first seven rotated components were found significant at the 0.05 or lower levels. All accounted for 68% of the total variance of the original data assembly. The different groups of highly intercorrelated streams were defined by the patterns of map drawn for the rotated components loadings. Furthermore, an annual cycle analysis was applied to the same monthly streamflow data. Harmonic vectors extracted from a series of 12 monthly means at each station were marked over a map to readily keep track of streamflow seasonal variations across the study domain. The basic statistical characteristics of streamflow were also investigated within the identified regions; thus, some similarities and dissimilarities between the regions were documented. The identified streamflow regions were shown to be consistent with climate zones in Turkey defined by a recent study based on cluster analysis of temperature and precipitation data.