Nineteen new record species for Argentina belonged to Bacillariophyceae and Chlorophyceae Classes were identified as the result of the analysis of a total of 103 phytoplankton samples obtained from a 5-year study survey performed in 6 lotic systems from Buenos Aires province, tributaries of Río de la Plata river: Paraná de las Palmas river (1995-1996), Samborombón river, El Pescado, J. Blanco, Buñirrigo, Rodríguez and El Gato streams (1997-1998) and in a intensive study carried out in the last two systems (1999). The taxa are ilustrated, a range of physical and chemical water variables of the sampling sites in which each species was found is furnished, together with the environmental requirements, cited in the literature. The 16 % of the algae were periphytic or epilithic species, the 37 % belonged to slighty saline to brackish waters and three of them resulted pollution-tolerant species