Magnetostriction of synthetic polycrystalline magnetite and titanium-rich titanomagnetites has been measured from 25 ¿C to their Curie temperatures. These are the first reported measurements of magnetostriction above room temperature for titanomagnetites. The magnetostriction constants are measured with a recording rotating field magnetostrictometer, which uses a strain gage bridge technique and two rotating SmCo5 permanent magnets. Magnetostriction is recorded continuously as the sample is slowly heated and cooled in a fixed magnetic field. Measurements were made on sintered polycrystalline samples of magnetite, titanomagnetite (Fe3-xTixO4, x=0.0, 0.4, and 0.6), and an aluminum-magnesium substituted titanium-rich titanomagnetite (AMTM60). The thermal dependence of the polycrystalline magnetostriction constant λs(T) varies as (1-T/Tc)1.3 for titanium-rich titanomagnetites and as (1-T/Tc)0.9 for pure magnetite. The polycrystalline magnetostriction constant is well approximated by a power law equation in terms of the saturation magnetization, λs(T)∝Ms(T)n, where n=2.31 (TM0), 3.47 (TM40), 2.75 (TM60), and 2.80 (AMTM60). The magnetostriction results for AMTM60 are in excellent agreement with previous predictions based on domain observations. ¿ American Geophysical Union 1993