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Late second and early third gregarious nymphal instars of the desert locust (Schistocerca gregaria Forskal) were sprayed with enriched neem oil (0.2% and 0.4% azadirachtin/l diluant oil) at an application rate of 10 l/ha. A change of the colour from the black and yellow gregarious type to a pale yellow transient and a green solitary type was observed during the following 33 days. In the treated nymphs the proportion of pale yellow and green coloured hoppers was significantly higher than in the untreated control. The change of colour was less profound at high population density (500 individuals/cage) compared to low population density (150 individuals/cage). The proportion of nymphs with damaged antennae and legs mainly as a consequence of the treatment was higher in green coloured hoppers compared to pale yellow transient and black and yellow gregarious hoppers.