Individual linear growth rates were studied in the freshwater pearl mussel Margaritifera margaritifera from the Vuokinjoki River. Growth deceleration coefficients were shown to vary widely and differ significantly among individuals. The average value of the growth deceleration coefficient for the population is 0.069. The growth of mussels in the Vuokinjoki River is accompanied by three regular biorhythms. These biorhythm periods were roughly constant both through an individual's ontogeny and among mussels. Their average periods were 13.3, 6.4, and 4.0 years. The growth deceleration coefficient a depends on the age of the mussels. We discuss the possibility that the hydrological regime disturbance caused by the Kem hydropower plants' cascade construction could influence the growth rate.