Membrane-associated phosphoinositidase C activity has been identified in Dictyostelium discoideum using phosphatidylinositol 4,5-bisphosphate as exogenous substrate. Maximal activity was observed with 0.4 mM phosphatidylinositol 4,5-bisphosphate at pH 7.0. The enzyme was stimulatedby micromolar concentrations of free calcium with maximal activity at 100 μM.