Incidence of antibiotic resistance and of R plasmids and its dynamics was investigated in 1084 Salmonella strains isolated in the Danube River on Czechoslovak territory in recent years. From 261 strains belonging to various serotypes of Salmonella isolated at various sampling sites along the Danube in June 1972 to June 1973, 31 strains were found to be resistant to antibiotics and 24 (77.2%) carried R plasmids. The influence of waste waters from large cities (Vienna, Bratislava) in this respect was striking. No correlation was found between the frequency of occurence of resistant Salmonellae and the season of year. The results from the season 1972 to 1973 were compared with those obtained in 823 stock cultures of Salmonellae isolated from the Danube in 1967-68 and in 1969. The increase of incidence of antibiotic resistance and particularly of R plasmids in Salmonella strains from the Danube up to 1973 is striking.