This paper reports a study of bubble nucleation by fission fragments in superheated water. The experimental work was conducted using a small bubble chamber especially built for the program. The minimum superheat necessary for nucleation of visible bubbles by fission fragments (the threshold) was measured at temperatures between 380 and 440°F.Predictions of the threshold are based on comparison of the energy and linear energy transfer (LET) of fission fragments with the values required for bubble nucleation. Because of the variation in fission-fragment energy, the comparison is made on the basis of the median, 80’th percentile, and maximum energy and LET of the fragments present in the experiment.The data indicate that the LET comparison is the appropriate basis for prediction of the threshold. Using an empirically adjusted value of the LET required for nucleation, the calculated threshold agrees reasonably well with the data but becomes increasingly discrepant with increasing temperature. Reasons for deviation of the data from predictions are discussed, but a definitive determination cannot be made on the basis of the available data.