A transport assay has been developed for use in the investigation of 1-deoxy-D-glucose uptake in trypanosomes. 1-Deoxy-D-glucose has high affinity for the trypanosome sugar transport system (net influx Km = 4.03 +/- 0.42 mM; V = 0.052 +/- 0.005 mM X s-1. D-Glucose oxidation is competitively inhibited by 1-deoxy-D-glucose. However, we show that 1-deoxy-D-glucose is not a substrate for metabolism and that the competition occurs because of interaction at the transport system. D-Glucose competitively inhibits 1-deoxy-D-glucose influx.