Apparatus is described for the rapid determination of the cut-off frequencies,f1and fα, of transistors in the 20–1000 Mc/s range. Accurate measurements at these frequencies are made possible by the application of transmission-line techniques to the method of comparing the high-frequency voltages which appear across small resistors connected to two leads of the transistor. Methods are adopted which separate the measuring circuits from the input circuit and make the design of the latter non-critical. The relative accuracy off1andfαmeasurements is discussed, and it is concluded that the inherently more accuratef1measurement should have an error within ±5%, whereas the error in fαis probably 2–3 times higher. A few typical measurements are given.