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The effect of Nosema locustae infection on the aggregation behaviour of the oriental migratory locust, Locusta migratoria manilensis, was studied using a two-choice arena olfactometer and electroantennography (EAG). Infected locusts had low antennal sensitivity and aggregation responses to faecal extracts and to locust body volatiles. Infected fifth instar nymphs had significantly lower aggregation index than the uninfected nymphs, although with fourth instars the effect on aggregation behaviour only occurred in infected females. With regard to antennal receptor sensitivity, infected adult locusts had significantly lower EAG amplitudes in response to extracts from faeces of the adult males. The effect was most pronounced in female locusts. In contrast, there was no significant difference in the EAG responses between the fifth instar male and female nymphs. Further analysis revealed that EAG responses of fifth instar nymphs and adults infected with N. locustae to the faecal extract and volatiles from fifth instar solitary-reared nymphs were not significantly affected. In contrast, infected nymphs and adults had remarkably low EAG amplitudes for the remaining stimuli.