C Ciliwung river stat from the telaga Mandalawangi Montain until into yhe Jakarta bay. Along the river flows, there are many activities of population which consists of agriculture, industry, trading, and Pejompongan PAM Intake, if there's a lack of management, thus, the quality of the water will decreased, considering the magnitude of Ciliwung River, thus, this research was conducted towards the quality of Ciliwung River water by observing and using phytoplankton as bioindicator. The objectives of this research is to observe the quality and the condition of river water by analyzing the biological parameter such as abundance, diversity index, evenness index, domination index, and saprobity coefficient, as well as acknowledging the bundance distribution model. This research was conducted on Juli - september 2005 and 10 stations which were divided into 3 segmens. Sample analysis were held by using the direct count method, tracking from the sirface by use Sedgwick Rafter Cell. the result on the first periode obtained 4 division that consist of 42 species, such as 12 species from the Cyanophyta, 25 species from Chlorophyta, 4 species from Chrysophyta and i species from Englenophyta. Thus, from the second periode, obtained 4 division that consist og 45 species, such as 8 species from the Cyanophyta, 28 species from the Chlorophyta, 5 species from Chrysophyta and 4 species from Euglenophyta division. the measurements most appropriate models is Motomura model. The measurement of Evenness Indexs (E) resulted a different result on the first and second periods, where on the firstperiod, the condition of ecosystem categorized in the unstable and stable categorization, while on the secon period, the of ecosystem categorized in the stable condition. On the diversity indeks (H) showed that on the first and the second periods, the water condition of Ciliwung river was categorized in the polluted categorization. Domination Indeks showed that the first and second period, the type of Phytoplankton was dominted by Cyanophyta and Chlorophyta claases, which showed the fertilization level of Olygotrophic. based on the Saprobity coeficient (X) it was resulted that Ciliwung river that Ciliwung river was categorized ny organic and anorganic materials. From the research result it is evidences that phytoplankton was less approprite to be utilized as the bioindicator on a highly low river, which could be seen on the first and second station on thenarrow side of the Ciliwung river that flow quit heavy, phytoplankton was perceived appropriate to be utilized as the bioindicator on the station 3 - 10, where on those station, the flow was decreased. In order to maintain the water of Ciliwung river, could be done by periodically and continously observing the river, determines the limit of areas and river area.