The effects of crowding and isolation on body color and behavior were observed for the mid-instar nymphs of the desert locust, Schistocerca gregaria. Some of the solitarious (isolation-reared) nymphs that were crowded for 1 or 4h during the third instar developed black patterns at the fourth instar, but most individuals remained unaffected. Black patterns appeared in all individuals that were crowded for 1day or longer, but even after 4days of crowding the black patterning for some individuals was not as intense as that for the gregarious (crowd-reared) controls. Isolation of gregarious nymphs caused the black patterns to recede or disappear at the last (fifth) nymphal instar, but it was necessary to isolate the nymphs from the beginning of the first instar to obtain body coloration looking like solitarious nymphs in most individuals. Solitarious nymphs that were allowed to see gregarious nymphs developed different intensities of black patterns depending on the body size and number of nymphs shown. The behavioral phase shift from one phase to another was observed when the nymphs were crowded or isolated for 2days or longer, as previously reported for the last nymphal instars of the same strain. Behavioral gregarization was induced for isolated nymphs that were allowed to see a group of nymphs through a transparent double wall. These results suggested that body-color phase shift occurred more rapidly for mid-instar nymphs than for late instar nymphs but the rate of behavioral phase shift was similar for the two instars.