Pyromagnetic measurements of the magnetization of a single-crystal sphere of Permalloy (79.65 Ni-20.35 Fe by weight) in the range 4.2°-100°K and with applied fields of 4, 9, and 14 kOe parallel to the  crystal axis are reported and interpreted in terms of spin-wave theory. The spin-wave energy gap obeys the theoretical expression in terms of internal applied field, magnetocrystalline anisotropy field (negligible), and average spin-wave demagnetizing field. The coefficients appropriate to zero gap are C = (7.6±0.1)×10-6 deg-3/2 for the T 3/2 term and D/C≤±0.2×10-3 deg -1 for the T5/2 term. The analysis of the data is made using a new graphical method which displays visually the quality of fit to the theory. The C value implies (JS)eff/kB=134°K in good agreement with independent determinations of the spin-wave dispersion parameter. © 1964 The American Institute of Physics.