The effective communication of alarm can be critical for social animals so that they are able to deal with threats posed by predators and competitors. In the case of many of the most ecologically dominant, large-colony ant species, these alarm responses are aggressive and coordinated by alarm pheromones, produced generally from the mandibular glands. In the present study, the alarm behaviour of two Neotropical army ant species is examined, the swarm raiding Eciton burchellii (Westwood) and the column raiding Eciton hamatum (Fabricius). Both species exhibit aggressive alarm responses in response to crushed heads, suggesting that the alarm pheromone is indeed produced by the mandibular glands in these ants. The most abundant component of the mandibular gland secretion, 4-methyl-3-heptanone (10 µL on a rubber septum), stimulates a substantial alarm response, although this is less than the response to a single crushed head. This suggests that 4-methyl-3-heptanone may be an alarm-stimulating compound in Eciton. The alarm response of E. burchellii involves more workers than that of E. hamatum, although major workers play a much greater role in the response of the latter species. The differences in the alarm response of the two closely-related species may relate to their foraging strategies, with E. burchellii relying more on quantity rather than the caste of ants responding and possibly using alarm pheromones for offensive as well as defensive functions.