An assay is developed in which the behavioural responses of an individual locust to a group of conspecifics are used to derive an index representing the `behavioural phase status' of that insect. The index is generated from logistic regression models based on 11 behavioural parameters. Data are presented for locusts reared under crowded conditions for many generations and in isolation for one, two and three generations. Crowd-reared locusts differ in behaviour from insects reared in isolation in several ways. They tend to be attracted by a group of locusts, whereas insects reared in isolation are repelled. Solitaryreared locusts also show a characteristic set of behavioural responses which are all consistent with a cryptic lifestyle. Such responses become more evident with increasing number of generations of rearing in isolation. These data represent the most integrated behavioural analysis of locust phase differences yet undertaken. The procedure will be used in future work to quantify the dynamics of gregarization in congregated solitarious locusts. This will provide a much-needed basis for studying underlying controlling mechanisms, the nature of which is at present unclear.