A very large forced magnetostriction was observed for Y2Fe17and Lu2Fe17hexagonal single crystals near their ordering temperatures. The isotropic exchange coefficients are large, compared with the anisotropic terms. These results, which are consistent with the large magnetic volume anomaly observed previously, are interpreted in terms of the dependence of the magnetic interactions on interatomic distance. The largest magnetostriction is measured along the c-axis and a positive sign is observed for (c/a)/(c/a). Both these results are associated with the strong dependence on interatomic distance of the negative interactions occurring in the substitution zones.