The chemotaxis assay is used to study C. elegans olfactory behavior. The odorant or chemical of study is applied to one side of the agar plate, with a control chemical applied to the opposite side. Sodium azide is added to both sides to immobilize the worms upon reaching their chosen odorant. Worms are placed equidistant from either chemical and allowed to chemotax, or roam freely, for a set period of time (roughly an hour to ensure the sodium azide immobilizes all worms). A chemotaxis index is calculated by subtracting the number of worms on the control zone from the number of worms on the odorant zone, divided by the total number of worms. Therefore, a chemotaxis close to 1 indicates a strong preference for the odorant, while a value close to -1 indicates either a strong repulsion to the odorant, or a lack of preference in general. This assay is also applicable to Drosophila larvae, and can be adapted to study more complex behaviors such as associative learning.