The following expression is derived expressing the instantaneous growth rate (G) in terms of length (l) the power of the weight–length relationship (b), and the Bertalanffy growth parameters (KandL∞):G = bK(L∞/l − 1). Since this is valid for a length (or age) range in which growth conforms to the Bertalanffy model, a plot of G vs. 1/lshould be linear with the intercept on theGaxis being −bKand on the 1/laxis being 1/L∞. Since the variables can be measured independently, deviations of points from the regression can be tested and the limits of validity of the model ascertained. In addition, confidence limits ofKandL∞can be estimated. Two examples compare results with those using previous methods.Key words: method, Bertalanffy, growth, statistical, estimates, parameters