A straightforward method for calculating selectivity coefficients (Wij) of predation from raw data, mortality rates of prey, filtering rates, feeding rates and electivity indices is derived. Results from a comparison of selectivity coefficients for the copepod Diaptomus oregonensis grazing under a number of experimental conditions suggest that Wij's for size-selective feeding are invariant, a conclusion also supported by the leaky-sieve model. Recommendations are made on how to use Wij's in linear and nonlinear feeding constructs for zooplankton and other animals.