Introduction The cellular slime mold Dictyostelium discoideum is a widely used system for the study of cellular differentiation. In the so-called vegetative stage the free living amoebae feed on bacteria and divide by binary fission. However, when the food supply is exhausted, the cells aggregate under control of the secreted chemo-attractant, cyclic AMP, and a developmental program is initiated. The population of undifferentiated cells gives rise to two types of differentiated cells: stalk cells and spores, forming fruiting bodies. Several diffusible molecules (cyclic AMP, ammonia, differentiation inducing factor and adenosine) which control differentiation have been identified (Williams 1988). In addition, experiments under different environmental ionic conditions suggest a role for ions (Marin and Rothman 1980; Maeda 1983; Town 1984) and intracellular pH (Gross et al. 1988; Inouye 1988; Van Lookeren Campagne in press) in differentiation of D. discoideum cells. However, the specific functions of ions in differentiation have remained largely unclear. We study the electrophysiological properties of Dictyostelium (Van Duijn et al. 1988; Van Duijn and Vogelzang unpublished results) in order to examine the role of ions and ion fluxes in signal transduction and differentiation. To explore the role of ionic channels in the control of differentiation we studied the effect of potassium channel blockers on differentiation ofD. discoideum.