Abstract: The seasonality of stream flow variability indicates special feature of local cycle of precipitations, evaporation and the timing of snow melt. This study presents seasonal occurrence of maximal and minimal annual flows and spatial variability of seasonal index (Is) in the Middle and Lower Danube basin. The analysis is based on 47 time series of monthly runoff (12 for the Danube and 35 for its tributaries), which are collected from public database. The results show that the maximum annual stream flow appears during all months, but with highest frequency in April for 68% from watersheds. It varies between one and 60% (Danube – Baziaz). The monthly flow is concentrated in summer–autumn hydrological season, except for the Jalomita, Siret and Prut river basins, where it is in the winter. The highest frequency of minimum monthly runoff for the Danube and the Tisza is in October and November and for the Sava and Velika Morava – in August and September. The lowest and highest monthly discharge of the given month in the entire period were recorded in different years. Seasonal index (Is) is between 0.98 and 3.18. It is about 1.00 for the Danube and more than 2.00 for the Tisza, Ialomita, Siret and Prut watersheds. Is is stability – coefficient of variation is up to 0.30 with the exception of several river basins. Stream flow variability of the Middle and Lower Danube can provide valuable information for scientific studies and integrated management of water resources.