Helicity is the only invariant of incompressible flows whose derivative is continuous in C 1 -topology Let Q be a smooth compact orientable 3-manifold with smooth boundary ∂Q. Let B be the set of exact 2-forms B ∈ Ω 2 (Q) such that j * ∂Q B = 0, where j ∂Q : ∂Q → Q is the inclusion map. The group D = Diff 0 (Q) of self-diffeomorphisms of Q isotopic to the identity acts on the set B by D × B → B, (h, B) → h * B. Let B• be the set of 2-forms B ∈ B without zeros. We prove that every D-invariant functional I : B• → R having a regular and continuous derivative with respect to the C 1 -topology can be locally (and, if Q = M × S 1 with ∂Q = ∅, globally on the set of all 2-forms B ∈ B• admitting a cross-section isotopic to M × { * }) expressed in terms of the helicity.