The freshwater scud Echinogammarus stammeri is a very abundant benthic crustacean in riverine systems of northern Italy. The molecular physiology of L-[3H]leucine transport across cell membranes of E. stammeri was investigated. At pH 7.4, the presence of a NaCl gradient stimulated the leucine transport rate, when compared with KCl gradient. By contrast, alkaline pH strongly inhibited transport activity. In NaCl medium at acidic pH, leucine uptake displayed a specific activity 2.1-fold lower than that measured at neutral pH. Analysis of the uptake as a function of external leucine concentration revealed saturation kinetics. Leucine uptake appeared broadly unspecific and it was much strongly inhibited by the non-polar amino acids phenylalanine, methionine, and BCH, as well as by hydrophobic hydroxyacids. These results revealed important differences with respect to amino acid transport systems described in marine crustaceans. Their relevance in the feeding adaptation of the scud is briefly discussed.