Biomass and species composition of phytoplankton in relation to the physical and chemical conditions of the water in Lake Adale were studied from March, 2011 to September, 2011. All the physical, chemical and biological parameters measured varied temporally. The temperature of the lake was measured by digital oxygen meter and varied from 22°C to 26°C. The chemistry of the lake was basically similar to the dilute East African lake waters, with maximum pH values of 9.2 as measured in situ by a portable digital pH meter. The inorganic nutrients, which were of moderately high levels, varied temporally as a function of hydrological, hydrographic and biological conditions of the lake and were measured by a Hach kit (DR/2000 spectrophotometer) for nitrate, and colorimetrically with the ascorbic acid method for phosphate and molybdosilicate method for silica. The phytoplankton community, which was consistently dominated by cyanobacteria, exhibited low species diversity. Phytoplankton biomass measured as chlorophyll a varied from 23.35 to 55.18 mg m-3 .The concentration of chlorophyll a was calculated according to Talling and Driver (1963) using absorbance measurements made at 665 and 750 nm. The biomass of the lake indicates that the lake is a productive one. The factors responsible for the observed temporal variations in the physical, chemical and biological features of the lake are discussed.