Abstract: Monte Carlo simulation experiments were conducted to explore how variability in the maturity schedule for English sole Pleuronectes vetulus is transformed into variability in estimates of F35%. Random values were generated for age at 50% maturity and the maturity slope coefficient for each of 10935 combinations of 8 factors. From these simulated data F_35% values were derived and summarized for each experimental treatment as an average F_35% value and coefficient of variation. The summary statistics were then transformed and analyzed using forward stepwise linear regression to determine the relative importance of different factors plus a quadratic term were required to account for 90% of the variability in the data, whereas in the model for coefficient of variation in F_35% all eight factors, plus interaction term and a quadratic term, were required to account for 90% of the variability in the data. In both models the age at 50% maturity and the age at 50% selection were the two most important explanatory variables.