ABSTRACT.1Pheidole militicidaWheeler, a seed‐harvesting species of the southwestern United States, possesses a major worker caste (soldiers) with unusually large heads. Previous work suggested that these large major workers are specialized defenders against large seed‐harvesting species in the genusPogonomyrmex.2Experimental introductions ofPogonomyrmex maricopaWheeler and the omnivorous fire ant,Solenopsis xyloniMcCook, demonstrate thatP. militicidacolonies alarm‐recruit major workers againstS.xyloniworkers but not against the largerPogonomyrmexworkers.3P.militicidais the second species in which enemy‐specific defence againstSolenopsisfire ants has been detected. I suggest that the special role ofSolenopsisas both competitor and predator in ant communities frequently favours major worker specialization against this genus.4Major workers ofP.militicidaandPheidole dentataMayr, the other species with enemy‐specific defence againstSolenopsis, are morphologically very different from one another. It is suggested that interspecific variation in major worker morphology is not necessarily associated with variation in behavioural speci