In Lake Zürich, populations of the cyanobacterium Planktothrix rubescens develop in the metalimnion during the summer and become gradually entrained in the deepening surface mixed layer during the autumn. It had previously been demonstrated that the daily integrals of photosynthetic production accounted for the growth observed in the metalimnion and greatly exceeded the smaller increase during the autumn. We have now determined the relationship between growth rate (μ) and irradiance (I) in cultures of P. rubescens strain Pla 9316 maintained at 20 °C on a 12[ratio ]12 h light[ratio ]dark cycle: the highest net growth rate averaged over the 24 h (μh) was 0.123 d−1, the dark rate (μD) was −0.020 d−1, the gross rate (ϕm = μh−μD) was 0.144 d−1, the affinity coefficient (α) was 0.0273 (d μmol m−2 s−1)−1 and the compensation point (IC) was 1.76 μmol m−2 s−1. Using the corresponding coefficients calculated for the light period (μLh = 0.267 d−1, ϕLm = 0.287 d−1 and αL = 0.0547 (d μmol m−2 s−1)−1), instantaneous growth rates could be calculated from the irradiance. Comparison with growth rates at 10 °C indicated a Q10 of 1.48. These coefficients were used in a modification of the Smith equation to calculate potential growth rates of Planktothrix from the irradiance and temperature at each time and depth in Lake Zürich. Data on irradiance, vertical light attenuation and temperature were used to calculate the daily integrals of biomass increase over a period of 136 d. These growth integrals gave a closer correspondence to the observed population increase than the photosynthetic integrals calculated previously from measurements made with lakewater samples dominated by Planktothrix. Photosynthetic measurements made with the Planktothrix culture indicated a maximum rate of carbon increase (0.467 d−1 that exceeds the maximum growth rate, which suggests that other factors limit growth over long periods.