Non-linear distortion in junction transistors at low signal levels is investigated over a range of frequencies extending up to about 0.2 times the cut-off frequency. Theoretical expressions are developed for the second- and third-harmonic distortion as a function of d.c. conditions, frequency, and load and generator impedances. An alloy-junction transistor is studied under a range of different working conditions, the calculated results then being compared with those of experimental measurements.