For thermal convection between horizontal plates a certain Rayleigh number greater than the critical value must be attained before well‐defined, unsteady temperature fluctuations occur. Their appearance is found to be a strong function of Prandtl number, and has apparently been explained for large Prandtl number by a theory of Howard. These fluctuations at a given point midway between plates are intermittent in appearance over a considerable range in Rayleigh number. The Rayleigh number at which the intermittency essentially ceases and irregular fluctuations are nearly always present is determined and also found to be strongly dependent upon Prandtl number. There is no apparent association between the development of these short‐period fluctuations and the heat‐flux transitions of Malkus.