The causes of interspecific differences in the μ-l relationship are examined in the context of a mechanistic model which relates μ to irradiance in terms of six factors:φ, kc photosynthetic quotient (PQ), Chl a:C, respiration and excretion. The effect of cell size on the light saturated growth rate is also considered. It is shown that photosynthetic efficiency and PQ exhibit remarkably little interspecific variability, and average 0.024 ± 0.005 μg C(μg Chl a)−1 h−1 (μE m−2 s−1)−1 and 1.5 ± 0.2 mol 02 mol C−1 (when NO3− is the nitrogen source) respectively. Two useful relationships were derived: (i) between growth efficiency, αg and Chl a:C at μ. = 0; (ii) between the compensation intensity, Ic and the Chl a-specific maintenance respiration rate. Both relationships were independent of temperature and daylength. Species best adapted to growth at low light were found to exhibit high Chl a:C ratios and low maintenance respiration rates. As a group, diatoms were consistently the best adapted for growth at low irradiance. Chiorophytes, haptophytes, chrysophytes and cryptophytes were intermediate in their performance at low irradiance. Dinoflagellates exhibited extreme diversity, with species spanning the spectrum from very good performance at low irradiance to very poor. A new μmax-cell carbon relationship is given based on growth rates normalized to 15°C. Evidence is presented to show that noise in this relationship can be significantly reduced by using only carbon-specific growth rates and using only data for species grown at the same daylength.