The first determination of the complete base profile shape in a pushed‐out diffused transistor is reported. The effect of phosphorus emitter diffusion on a previously diffused gallium base has been monitored by radiotracer profiling using the isotope Ga67, and the phosphorus profile has been simultaneously measured by resistivity profiling. Definitive evidence is presented that the push‐out effect is caused wholly by a real inward migration of base impurity atmos at a rate which is much enhanced over the normal migration rate. The profile shape of gallium after push‐out shows a marked dip in concentrationwithinthe diffused emitter.