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During the act of egg laying, in the desert locust Schistocerca gregaria (Forskal 1775), females cover their eggs with a substance originating from the accessory glands. It hardens into a foamy material used as a chimney through which the delicate eclosed larvae can climb to the surface. The foam contains an as yet unidentified factor that promotes gregarisation. To distinguish the effect of this factor from that caused by another gregarisation promotory agent, namely the neuropeptide [His(7)]-corazonin, a comparative developmental study was undertaken. Egg foam significantly influenced the development of both crowd- and isolated-reared hoppers. The development was longer if the eggs were washed and the factor was absent. The morphometrics were only slightly affected, and there was no effect on the coloration of the locusts.