Temporal and spatial variations of phytoplankton primary production in Yenicaga Lake were evaluated by monthly in situ observation from September 1998 to August 1999. Primary production rates were estimated by determining the C-14 uptake rates of phytoplankton using radioactive sodium bicarbonate ((NaHCO3)-C-14). The highest primary production was measured at the water surface in August 1999 (455.6 mg C m(-3) h(-1)), coinciding with the period of the greatest phytoplankton bloom. The lowest primary production occurred in 2 m depth in April 1999 (0.91 mg C m(-3) h(-1)). The daily primary production ranged between 60 and 6202 mg C m(-2) day(-1).