Both. Imported fire ant workers bite (with mandibles) and sting (with stingers on the end of the abdomen) aggressively and repeatedly. See also FAQ #850: "What should I do if I get stung by an imported fire ant? How do I treat the pustules?" and FAQ #849: "What if I have an allergic reaction to a fire ant sting?"
FAQ #1096
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Do fire ants bite or sting?
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