1. The effect of cell density on chemotaxis in the cellular slime mold Dictyostelium discoideum has been examined by placing attracting and responding populations of washed myxamoebae at various densities on a hydrophobic agar surface. Aggregations in the former populations attracted cells out of neighboring responding populations which were deposited a few hours later. Attracting populations of 400-5000 cells, confined in droplets with a diameter of 0.5- 0.7 mm., all induced a similar chemotactic response. 2. The period between early aggregation in the attracting populations and its completion was unchanging at the various cell densities. 3. Smaller populations with a diameter of ca. 0.35 mm. and containing ca. 500 cells gave a maximal secretion of attracting substance per cell when compared with cells at higher densities. At lower densities, the attraction by the whole aggregate was decreased. The results show that very small groups of cells optimally spaced are able to attract all myxamoebae within an area normally covered by an aggregation of D. discoideum.