In the vegetative state, Dictyostelium amoebae are chemotactic toward pterins released by bacteria, whereas during multicellular development, they become chemotactic to endogenously produced cAMP. A variety of assays have been used to visualize and quantify chemotactic movement. Under-agarose chemotaxis provides a simple and flexible assay that permits high-resolution imaging and quantification of the motility behavior of individual cells and populations by both transmitted light and fluorescence microscopy. The assay requires cells to deform a solid but flexible matrix; therefore, it also provides a way to measure defects in the ability of mutant cells to move in these restrictive conditions.