A mutant (TS-2) that is temperature-sensitive with respect to cell contact was isolated from the cellular slime mold, Dictyostelium mucoroides. The TS-2 were able to grow and develop normally at 21°C, but unable to grow at 31.5°C. When TS-2 were allowed to develop until the aggregation stage at 21°C and then shifted to 31.5°C, they instantly lost cell-to-cell contact, resulting in disintegration of the aggregation stream and flattening of the aggregation center. Although a slug transferred to 31.5°C retained its original shape, loss of cell-to-cell contact within the cell mass was evidenced by several facts. The TS-2 interphase amebas, at 31.5°C, also lost cell-to-substratum contact, and the loss of contact was followed by the production of cell-wall substance on their surface. The production of the same substance at 31.5°C was also observed in cells at aggregation and migration stages, but not in those at the vegetative stage. When TS-2 cells at various developmental stages were kept at 31.5°C for various periods of time and returned to 21°C they lost morphogenetic capacity in proportion to the production of the cell-wall substance.