As a part of water quality monitoring involving 7 sampling sites on the Danube river within 1869.2-1768.0 river km the abundance of actinomycetes and seasonal variations were investigated. the highest number of actinomycetes up to 230 CFU/ml were found during early spring as a consequence of melting of snows. Minimal contents of streptomycetes approximately 1/10 of the peak values were found during late spring and summer season from may to september. the abundance of actinomycetes in this part of the year does not correlate neither with high discarge values of the Danube caused by intense summer rains nor with higher temperature of water. the second peak values up to 200 CFU/ml occurred during october-november. Sedimentation processes in the water reservoir formed by the Hrušov dam as a part of Gabčíkovo hydropower plant causes drastic decrease in actinomycete abundance in the old Danube bed.