Dictyostelium discoideum exists as single amoeboid cells during the first phase of its developmental cycle. These cells phagocytose bacteria. Chemo­ taxis to compounds released from the bacteria (e.g. folic acid) is probably involved in food seeking (95). After the growth phase the amoebae differen­ tiate into cells capable of aggregating into a multicellular organism. A breakthrough in the investigation of Dictyostelium chemotaxis was the discovery in 1967 that aggregating cells of D. discoideum are attracted by cyclic AMP (cAMP) (64). Normally in the laboratory D. discoideum devel­ ops asexually, but by combining cells of different mating types sexual devel­ opment can be induced. Chemotaxis is also involved in sexual development as the cells are attracted by immature macrocysts (93). These are cell groups in which zygotes are formed. In the multicellular stage the cells differentiate into spores and cells forming a stalk and basal disk. These two types of cells constitute the fruiting body formed at the end of development. Evidence has been presented for an action of cAMP in the sorting out of pre-spore and pre-stalk cells, suggesting that the spatial pattern of these cells is determined by chemotaxis (88). Chemotaxis to oxygen seems also to be involved in aligning the pre-spore/pre-stalk pattern in the stage preceding fruiting body formation (114). The discovery that cAMP is a chemoattractant established cAMP as a primary, extracellular messenger. Cyclic AMP exerts its function as a che­ moattractant by binding to cell-surface receptors (47, 49, 70, 78). In addi­ tion, cAMP affects cell differentiation (16, 18, 35) and stimulates its own