1) Locusta migratoria migratorioides (R. & F.) hoppers reared in crowds march actively; those reared in isolation do not, even when placed in a marching crowd. When rearing conditions are changed the amount of marching alters from one extreme towards the other. 2) Significant changes in marching occurred within 15 days of altering rearing conditions, well within the nymphal life of each locust. 3) It is suggested that long term changes in behaviour are associated with changes in endocrine status.