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Studies of the relative importance of various phytoplankton size classes (pico-, nano-, micro-, and macro-) during thermal stratification at oligomesotrophic Mountain Lake, Giles County, Virginia were conducted from March through October 1994 . A detailed time-and depth-series study was conducted to compare 14 C-fixation rates in these phytoplankton size fractions . Data indicated that on average the microplankton were the most important size class in cell number and cell volume and contributed ,:J95% of the total primary productivity . The picoplankton ranked second in importance constituting ;z:5% of total primary productivity, whereas the relatively small numbers of nano-and macroplankton contributed little primary productivity . No obvious relationships were apparent among cell densities, cell volumes, and chlorophyll a values during late summer thermal stratification, possibly due to shifts in community structure at that time . The findings contrast with many other oligo-to mesotrophic ecosystems where the pico-and nanoplankton have demonstrated significantly greater importance, especially in summer .