Worker insects altruistically sacrifice their own reproduction to rear non-descendant kin. This sacrifice reaches its most spectacular level in suicidal colony defense. Suicidal defense, such as when the sting of a honey bee worker embeds into a predator and then breaks off, is normally a facultative response. Here we describe the first example of preemptive self-sacrifice in nest defense. In the Brazilian ant Forelius pusillus, the nest entrance is closed at sunset. One to eight workers finish the job from the outside and, in doing so, sacrifice their lives.