From a quantitative perspective, it reveals two maxima of phytoplankton development, the first peak in June - July and the second in the month of September - October (Luiza F., 2008). Quantitative data on the structure of zooplankton varies from one basin to another and even from the same basin for the same period in different years.Sampling and data interpretation was carried out from May to September 2006 in three sections and a total of five stations, as follows:• Sections I - Rădăuţi (km 652) – Stânca - Costeşti (km 544): Station 1 P - Rădăuţi.• Sections II - Stânca-Costeşti (km 544) - Drânceni (km 284): Station 2 P – Ungheni and Station3 P - Drânceni.• Sections III - Drânceni (km 284) - confluence Dunăre: Station 4 P – Fălciu and Sta-tion 5 P - Oancea.Hydrobiological investigations in 2006 were aimed at quantitative determination of phytoplankton and zooplankton in the three sections of the Prut River. he quantitative structure of phytoplanktonStations established on the Prut river, the highest density was recorded in May at sta-tion 3 (Dranceni) - 1008 ex / ml, this period is characterized by heavy rainfall and lowest densities were recorded in June station 1 (Oancea), 136 ex / ml.From a quantitative perspective, we see that the weight of biomass is generally propor-tional to the number density. Inconsistencies appear during certain periods between propor-tions of phytoplankton biomass and densities of. This is due mainly forms whose weight is determined not so much by their quantitative development as their larger size. The biomass Prut River registered values between 0.068 to 3.7 mg / m3.The quantitative structure of zooplanktonThe total number of species has led an ascendant trend, showing a slight decline in September. The Prut river, recorded in May a maximum number density in station 5 (Oan-cea) - 846 ex / l and the lowest number density met station 3 (Dranceni) - 68 ex / l.Research results performed in period depicts a different aspect of time and space to develop quantitative zooplankton on the river Prut. The investigations made that rotifer dominate numerically and copepods and cladocerans own share in making biomass. Bio-mass is done on a whole of rotifers, copepods and cladocers follow. value of zooplankton biomass in the Prut River registered values between 0.16 to 3.15 g / m3.conclusionsQuantitative differences of phytoplankton analysis of the 5 stations, is due to the dif-ferent water conditions existing in that area, under the general effects of pollution on the structure and composition influences existing algae.Quantitative variations of phytoplankton are due: leak rate varies from one section to another, organic loading and water level.There is a close interdependence between qualitative and quantitative composition of zooplankton.