Phytoplankton is one of the parameters that determine the primary productivity in the sea. Distribution and abundance of phytoplankton is influenced by physical and chemical aspects of sea water. The purpose of this study is to analyze the abundance of plankton and chlorophyll-a. The research was conducted in November 2010 in the Tambelan Waters, Natuna. 24 species of phytoplankton were found. It was dominated by the group of Bacillariophyceae such as Bacteriastrum, Ditylum, Thalassiothrix, Hemiaulus, Nitzshia, Chaetoceros and Skeletonema. While the group of dinoflagellate species found are common and many of Ceratium. The content of chlorophyll-a showed that the range of chlorophyll-a in the Tambelan waters ranged between 1.71 - 4:08 mg/m3 (in the surface layer) and 0.92 - 5:39 mg/m3 (in the layer near the bottom). This condition indicates that the research are greatly influenced by the activity of the mainland. These results are also evident from the high concentrations of chlorophyll-a in the waters near the mainland compared to offshore areas, this confirms that the distribution of chlorophyll-a concentrations in waters strongly influenced by high nutrient inputs from activities in the surrounding land.