The values of the average number of prompt neutrons emitted in the spontaneous fission of some recently available even-even heavy nuclides have been determined by separately counting neutrons and spontaneous fissions and comparing these counts with similar counts from 252Cf standards. The results are as follows: 244Pu , 248Cm , 250Cm , 250Cf , 254Cf , all on the basis of for 252Cf. A comparison of these results with existing data for other heavy nuclides indicates that is both Z and A dependent in a manner similar to that calculated by Bondarenko et al.