Photosynthetic activity by phytoplankton was measured during the ice-free seasons of 1984, 1985 and 1987 using the ¹⁴C radioassay in high altitude Emerald Lake (California). Relative quantum yield (^B) and light-saturated chlorophyll-specific carbon uptake (P_m ^B) were calculated from the relationship of light and photosynthesis fitted to a hyperbolic tangent function. Temporal changes in P_m ^B showed no regular pattern. Seasonal patterns of ^B generally had peaks in the summer and autumn. Phytoplankton biomass (as measured by chlorophyll a) and light-saturated carbon uptake (P_m) had peaks in the summer and autumn which were associated with vertical mixing. Estimates of mean daily carbon production were similar among the three years: 57 mg C m^–2 2 d^–1 in 1984, 70 mg C m^–2 2 d^–1 in 1985 and 60 mg C m^–2 d^–1 in 1987. Primary productivity in Emerald Lake is low compared to other montane lakes of California and similar to high-altitude or high-latitude lakes in other regions.