The bulk lifetimes of minority ,carriers in n-type l :germanium, in both n and p-type silicon and in n-type magnesium germanide have been investigated at room temperature. The sample was illuminated with periodic light ·flashes produced by a spark gap. The transient change in conductance of the sample after illumination gave the lifetime. In germanium and silicon the measured bulk lifetimes had values from 15 to 140 microseconds with estimated errors of 4 to 15 per cent, respectively. It was not possible to measure the lifetime in magnesium germanide because the particular sample used in this experiment was not sufficiently pure. The general solution of the equation of continuity for the minority carriers in a .rectangular sample has been obtained. The particular solution for the boundary and initial conditions of this experiment is discussed.