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The size of defects necessary to cause pulsations, in otherwise ideal semiconductor injection lasers, is calculated for two defect models: a nonradiative region and a number of nonradiative surfaces. The point of instability for pulsations is found by solving the linearized rate equations describing small oscillations about the steady state and determining the length of the nonradiative region or the number of surfaces for which damped oscillations change to growing oscillations. The validity of this method is verified by computer calculations of large amplitude pulsations in a particular case. An oscillating light intensity induces an oscillating component of gain. Part of the oscillating gain is in phase with the light intensity in the absorbing region, causing growth, and is directly out of phase in the amplifying region, causing damping. The laser pulsates if the in‐phase gain in the absorbing region is dominant. This component of gain is enhanced by a short nonradiative lifetime. This mechanism applies over a broad range of parameters with optimum values of about 0.06–0.10 nsec for the nonradiative lifetime and 1–2 mW for the laser power. The requirement for pulsations found in earlier studies of divided contact lasers, that the gain versus carrier density relation g(n) be nonlinear with dg/dn greater in the absorbing region than in the amplifying region, is not required for pulsations if the relaxation time is short. However, increased dg/dn in the absorbing region greatly enhances the ability of a small defect to produce pulsations. For g(n) increasing linearity with n, a defect at least 11 μ long is needed to cause pulsations, whereas for dg/dn twice as great in the absorbing region as in the amplifying region, a defect only 4 μ long can cause pulsations. The possibility of increased dg/dn and absorption in the nonradiative region due to local heating is discussed. For optimum lifetime and surface recombination velocity,- each nonradiative surface is approximately equivalent to a nonradiative region about 1/2 μ in length. Accordingly, two nonradiative cleaved surfaces are insufficient to be the sole cause of pulsations.