In order to evaluate the strain rate sensitivity of dynamic flow stress of aluminum alloy at very high strain rates, constant strain rate tests and strain rate reduction tests are conducted for 7075-T6 in the strain rate range from about 1×10^3s^<-1> to 3×10^4s^<-1>. The flow stress increases steeply above about 2×10^4s^<-1>. The behavior is interpreted on the basis of a simplified model which can represent a transition in the rate controlling mechanism of dislocation motion from a thermally activated process to a viscous drag.