The motion of cells in the aggregation phase of Dictyostelium discoideum development is complex. To probe its mechanisms we applied precisely timed (±1 s) and positioned (±2 Am) pulses of cyclic AMP to fields of cells of moderate density using a micropipette . We recorded cell behavior by time-lapse microcinematography and extracted cell motion data from the film with our Galatea computer system . Analysis of these data reveals: (a) Chemotaxis lasts only about as long as the cyclic AMP signal ; in particular, brief pulses (-5 s) do not induce chemotaxis . (b) Chemotactic competence increases gradually from within an hour after the initiation of development (starvation) to full competence at -15 h when aggregation begins under our conditions . (c) Cell motion reverses rapidly (within 20 s) when the external gradient is reversed . There is no refractory period for motion . We present a new description of the process of aggregation consistent with our results and other recent findings . (d) The behavioral response to cyclic AMP includes a phenomenon we call "cringing." In a prototypical cringe the cell speed drops within 3 s after a brief cyclic AMP stimulus, and the cell stops and rounds and then resumes motion after 25 s. (e) The development of the speed response in cringing as the cells age closely parallels the development of the cyclic AMP-induced light-scattering response of cells in suspension . ( f) Cringing occurs in natural populations during weak oriented movement . The computerized analysis of cell behavior proves to be a powerful technique which can reveal significant phenomena that are not apparent to the eye even after repeated examination of the film .The cellular slime mold, and Dictyostelium discoideum in particular, has gained some notoriety as a system in which to study a range of phenomena including intercellular communication, chemotaxis, cellular differentiation, and the regulation of spatial patterns in development (6,16,32,56,66,81) . Detailed mathematical models have been developed for the aggregation process in this organism (12,13,66,67). One purpose of this paper is to furnish the type of detailed data necessary to evaluate the models. D. discoideum initiates development when starved . The cells (myxamebas) go through a period of interphase, aggregation to centers, formation and migration of a slug (grex or pseudoplasmodium), and fruiting body construction . During the interphase to aggregation period the cells move as individuals, colliding at times, so they can be filmed and the behavior of each cell analyzed. During this period the cells' behavior is coordinated on a field-wide basis-the cells behave as an integrated tissue.Cyclic AMP appears to mediate aggregation in D. discoi-THE JOURNAL OF CELL BIOLOGY " VOLUME 92 MARCH 1982 807-821 ©The Rockefeller University Press -0021-9525/82/03/0807/15 $1 .00 deum. Cyclic AMP (cAMP) signals from single cells or groups diffuse to neighboring cells which relay the cAMP signal and move toward the source by chemotaxis. We applied cAMP to...