International audience This paper documents the articulation of the interdental approximant, an unusual speech sound that occurs in several languages spoken in the Philippines and Western Australia. This sound is notable for the fact that the tongue protrudes from the mouth and contacts the lower lip, and it seems to have a lateral perceptual quality, but documentation of the other details of the sound have been sketchy. We use ultrasound imaging to study the sound produced by a speaker of Kagayanen. We show that the only constriction is interdental, that the degree of tongue protrusion is related to vowel context and focus, and that the sound does not involve tongue raising. Coronal section images indicate that the sound involves the lowering of at least one side of the tongue, making it articulatorily lateral. We also discuss the implications for theories of tongue movement.