Hardness of Crystals with NaCl Structure and the Significance of the GILMAN-CHIN Parameter*)The microhardness of PbTe, PbSe, EuS and EuSe has been determined. The microhardness vs lattice constant plot for chalcogenides is a curve similar to the one for alkali halides. The Gilman-Chin parameter (HIC,,) has been calculated for a large number of crytals with NaCl structure. There is no unique value representing the entire group of crystals, the values ranging from 0.01 to 0.20.The Gilman-Chin parameter broadly correlates with the effective ionic charge (e*) indicating that it is a measure of the ionicity. Empirically, it is observed that the product (HIC,,) . (e*)' is approximately constant.