We tested 2 sources of growth rate variability in a lotic species of unionoid mussel, Pronodularia japanensis, in an agricultural drainage channel in Japan. First, we examined the effects of handling (excavation with short-term aerial exposure). Mussels were tagged, released, and recaptured every 0, 1, 2, or 3 mo to measure growth rate. Growth rates of individuals were not affected by the frequency of excavation. Second, we examined the effect of P. japanensis density on the growth rates of juveniles. Juveniles (shell length <30 mm) were exposed to varying densities of adults in experimental cages in 3 different seasons. In early summer in habitats with slow currents, growth rate of juveniles was significantly and positively related to density of P. japanensis. Our results suggest that excavating unionoid mussels at intervals as short as 1 mo is unlikely to cause physiological stresses on individuals and that density-dependent effects on growth rates might be important only at certain times of the year.