A series of studies was made of bubble nucleation by fission fragments and fast neutrons. The experimental work was conducted by a liquid suspension method in a pressure chamber designed to provide for visual determination of the onset of nucleation. The minimum superheat necessary for nucleation of visible bubbles was measured in water and propylene glycol. An analytic prediction method for the superheat threshold is presented, utilizing the “thermal spike” theory of Seitz and Rayleigh’s criteria for instability of a vapor jet in liquid. This method predicts that the important parameter a, equal to the ratio of the track length in which net energy must be deposited to the critical bubble radius, should equal 6.07. By this analysis, this value is independent of the type of thermal-spike-producing radiation, the type of fluid, and the system condition. The experimental data from this investigation and all other published data were examined to determine the applicable a values. This examination did not result in identification of a values consistent with the proposed prediction. Reasons for the deviation of the data from predictions are discussed, but the basis of the deviations cannot be resolved.