During aggregation of Dictyostelium discoideum, amoebae move towards collecting centres emitting cyclic AMP. Previous work has indicated that Ca2+ can affect the operation of various parts of the chemotactic mechanism but there are contradictory reports about the role or requirement for Ca2+ during aggregation of intact amoebae. In this study we show that there is a requirement for Ca2+ during this aggregation process. Addition of EGTA (7 mM) and in some cases extensive washing with EGTA is needed to see inhibition of aggregation. Considerable stores of Ca2+ present within cells may explain the failure of some previous attempts to see a Ca2+ requirement during aggregation.