Residence time distributions for wheat starch in a single screw extruder were measured for different operating conditions using a manganese dioxide tracer. Two flow models which differed in the definition of dead space were used to represent the flow pattern in the extruder. The parameters for each model were fitted using the experimental residence time distributions. Screw speed and the moisture level of the feed showed the strongest effects on the average residence time. A small fraction of the flow, 5–20%, was held up in the dead space region. The remainder passed directly through the active region with a residence time distribution close to plug flow. An apparent power law index for the starch melt was calculated and over the experimental range of extrusion conditions it varied from 0.2 to 1.0.