Lake stability (S) calculations quantify the resistance a column of water has to vertical mixing. Data from a selection of New Zeland lakes have been used to show a predictive relationship between the seasonal maximum stability (Smax) and mean depth (z, volumersurface area). Because of their proportionality, Smaxcan be substituted for the total annual stability (Stot) for the purposes of lake comparisons, although Stotwould be a better ecological parameter if enough data were available. Stability calculations in the euphotic zones of lakes enable useful comparisons to be made regarding growth conditions for different types of phytoplankton, in particular buoyant cyanophytes, and especially in relation to light penetration. An example is given where the degree of thermal stratification (stability) could be related to wind stress energy.