The delay time involved in switching indium cryotrons is investigated as a function of the applied magnetic control fields. For overdrive fields greater than the isothermal critical magnetic field, but smaller than the constant-entropy critical magnetic field, the rate of transition appears to be controlled by the rate of supply of latent heat from the environment. In this range of overdrive, delay times (for build-up of full resistance) experimentally determined and calculated with overdrive and temperature as controlled variables are in the range of 0.1 to 1 μsec in good quantitative agreement. For overdrive fields 40 per cent above the critical magnetic field, the delay times (10 to 40 nsec) for build-up of full resistance are unexpectedly long compared to the phase propagation time through the film (less than 1 nsec) computed from Pippard's equations for eddy-current delay. Indeed, the normal phase appears to nucleate first in isolated regions from which it propagates laterally (i.e., in the plane of the film) to provide for a rather continuous increase in resistance from the time the pulse is started until the transition is complete. Thus, the major delay in the completion of the transition is probably associated with the time taken to propagate the phase between the various nucleation sites. The propagation distance, i.e., the distance between nucleation centers, is then calculated to be several microns. Methods of decreasing the delay time are discussed. © 1963, IEEE. All rights reserved.