Growth rates of populations of the freshwater pearl mussel, Margaritifera margaritifera (L., 1758), in northwestern Spain were analysed based on measurements of annual annuli and using two nonlinear functions for length-at-age data sets: von Bertalanffy's growth model and a hyperbolic function. These populations reach the smallest maximum shell length (90.5 mm) and have the shortest life-span (35 years) and the highest growth rate (k in von Bertalanffy's model >0.1·year^–1, on average) known for this species. The two models were similar in performance and were well fitted (around 99%) to shell-length-at-age data, although the hyperbolic function appears to be applicable only from 6 years of age. The growth rate (either k or k' from the hyperbolic function) showed a large and significant variation across populations, both among and within drainages.