In order to find out if head bug resistance of sorghum grains was related to grain hardness, a method was developed in our laboratory to evaluate the hardness of developing sorghum grains by usina the INSTRON Universal Food Testing Machine. Penetration tests, with a very sharp needle, as thin as an insect stylet, were carried out on individuel immature caryopsis collected twice a week from seven days aflcer anthesis through 28 days anthesis. The force in Newtons required to penetrate into the grain was measured. Third sorghum cultivars grown during two successive years (1991 and 1992) were studied - one normal (IRAT 202), one resistant (Malisor 84-7) and one susceptible (S 34) to head bug attack. Developing grain hardness was attributed to endosperm rather than to pericarp hardness. At the first development stages when the grains are more susceptible to head bug attack, Malisor 84-7 hardness increased more rapidly with maturity than S 34 one. A similar pattem was observed for dry master accumulation and percentage. Free phenolic compounds and tannin contents of immature caryopsis were also determined and their relationships to head bug resistance were discussed.