Magnetostriction measurements were performed on (1 1 0) discs using a strain gauge technique. The magnetostriction constants lambda /sub 100/ and lambda /sub 111/ as a function of temperature show that for Tb/sub 3/Fe/sub 5/O/ sub 12/ the lambda /sub 111/ is much larger t han lambda /sub 100/, while the inverse holds for Eu/sub 3/Fe/sub 5/O/sub 12/. Also, lambda /sub 111/ of Tb/sub 3/Fe/sub 5/O/sub 12/ proved to be quite large (+5.6 x 10/sup -4/ at liqui d N temperature) and showed a rather sharp increase at low temperatures. The Tb/sup 3+/ electronic level energy shift was calculated. A mechanism is proposed for the dissipation of microwave energy. (D.C.W.)