The paper presents results of a comparative analysis of two mathematical models of fish length growth: the von Bertalanffy equation and the modified power function. The accuracy of mathematical modelling of empirical data and a potential for extrapolation of theoretical growth range beyond the empirical data are compared. The data on accuracy and extrapolation potential, obtained with the two models tested and averaged for a species, were subject to the Student′s t test. Additionally, an attempt was made to determine a relationship between accuracy and extrapolation potential of each model; linear regression was used to describe the relationship. The generalised (ignoring between-specific differences) description of the relationship involved exponential, logarithmic, and power regressions in addition to linear regression.