A canavanine-resistant mutant strain, defective in the transport of arginine and ornithine, was isolated and characterized. Experiments presented show that both the kinetics of influx and the steady state of accumulation of arginine and ornithine are affected by the mutation, whereas the activity of other related transport systems remains unchanged. On the basis of competitive studies, it is concluded that L-canavanine can inhibit efficiently the arginine-specific uptake system. D-Arginine appears to be a moderate inhibitor. None of the basic amino acid-binding proteins of the mutant strain showed detectable alterations in terms of quantity, physical properties, or affinity constants. Studies on the relationship between the number of transport carriers and the steady state of accumulation of arginine suggested the presence of a reduced number of membrane carriers in the mutant strain. It is proposed that the mutation affects a regulatory gene concerned with controlling the amount of membrane carriers produced, which are components of the arginine- and ornithine-specific uptake systems. The mutation maps at min 62 on the recalibrated linkage map of Escherichia coli K-12, in a locus closely linked or identical to argP.