Abstract In five annual periods, on Negros Island in the Philippines, nomadic behaviour was investigated in two surface-adapted species of the doryline ant, Aenictus . Twenty colonies of A.laeviceps and fifty of A. gracilis were studied for intervals ranging up to several weeks, one colony of the first species in round-the-clock observations for nearly two months and one of the second for one month. Results supported the hypothesis that emigrations in Aenictus have aspects of behavioural simplicity consistent with the monomorphic population conditions of broods and workers in these ants, as against behavioural specialization in Eciton and other polymorphic dorylines. Emigrations in Aenictus can begin relatively suddenly—usually within 20 min after the first signs of excitement at the bivouac—thus without the day-long preliminaries in raiding that characterize colony exodus in Eciton . In Aenictus these movements start at almost any time of night or day, with variations depending on colony condition. Although the common pattern is that of raids leading into changes of bivouac, emigrations can begin early or late in raiding, as actions overlapping previous emigrations, or as actions ending a quiescent interval without extra-bivouac group operations. A variable relationship between raiding and emigration is marked in Aenictus by frequent emigrations not correlated with raids either in their timing or in their route. New routes followed from the bivouac or taken beyond the zone of raiding, especially in the last half of the nomadic phase, may be those of other colonies. As this phase advances, emigrations typically increase in duration from around 2 to more than 6 hr, in distance from 20 to more than 60 metres, and their daily number to more than five. By contrast with the regular nocturnal emigrations of Eciton that follow prerequisite day-long raiding operations, emigrations in Aenictus seem to come promptly at the rise of intrabivouac excitation caused by stimulative activities of recently food-depleted larvae. The facile transformation from raiding or some other colony operation, distinctive of Aenictus , may represent a generalized, primitive condition of colony organization. Results for this army ant point to relatively direct communicative processes between larvae and workers, energizing actions of exodus promptly. The archaic conditions of monomorphic and relatively small populations evidently simplify group function so far as to greatly reduce the complexity and developmental time of colony actions including emigration. Reductions in the frequency of colony emigrations with sexual larvae—as against the normal all-worker broods—suggest qualitative differences in the stimulative effects of brood on the colony. After the colony queen has been removed in a test, emigrations decrease greatly in their normal forward directionality. Emigrations—attributed to intra-bivouac excitation from brood—are contrasted with the shifts of bivouac that occur infrequently in the statary phase, marked by distinctively sluggish colony behaviour and attributed to extrinsic disturbances. Nomadism greatly increases the scope of colony predatism in Aenictus . One colony, in 46 emigrations carried out in 27 nomadic days, covered an ant-trail distance of more than 1200 m.