Terrell proposed, in 1959, a relationship between the mean energy of a fission spectrum and the average number of neutrons resulting from fission. Using the relationship between the mean energy of fission neutrons and the Maxwellian temperature of the fission spectrum, his relationship can be written as We have used a weighted least squares method to obtain values for a and b from available experimental determinations of Tm or its equivalent. The values we obtain are a = 0.353, b = 0.510. We have also fit the relationship obtaining values for c and d of 0.997 and 0.125, respectively. The goodness-of-fit criteria are essentially the same for the two representations. Terrell’s relationship was derived from theoretical considerations, but there is no theoretical basis for the linear representation.