Spray basil plants with an insecticidal soap or liquid seaweed labeled to control soft-bodied insects. Spray the underside of the leaves as well as the visible parts of the plant to eliminate all the aphids.
Introduce ladybugs to your garden. Ladybugs and their larvae feed voraciously on aphids; they can be purchased from garden supply centers.
Plant companion plants that are known to repel aphids near your basil. Garlic, onions, coriander, fennel and French marigolds all control the pests. Since garlic and onions may flavor basil planted nearby, plant them only near crops intended for use in savory sauces and dishes.