a b s t r a c t Let (M, I, J, K ) be a hyperkähler manifold, and Z ⊂ (M, I) a complex subvariety in (M, I).We say that Z is trianalytic if it is complex analytic with respect to J and K , and absolutely trianalytic if it is trianalytic with respect to any hyperkähler triple of complex structures (M, I, J ′ , K ′ ) containing I. For a generic complex structure I on M, all complex subvarieties of (M, I) are absolutely trianalytic. It is known that the normalization Z ′ of a trianalytic subvariety is smooth; we prove thatTo study absolutely trianalytic subvarieties further, we define a new geometric structure, called k-symplectic structure; this structure is a generalization of hypersymplectic structure. A k-symplectic structure on a 2d-dimensional manifold X is a k-dimensional space R of closed 2-forms on X which all have rank 2d or d. It is called non-degenerate if the set of all degenerate forms in R is a smooth, non-degenerate quadric hypersurface in R.We consider absolutely trianalytic tori in a hyperkähler manifold M of maximal holonomy. We prove that any such torus is equipped with a non-degenerate k-symplectic structure, where k = b 2 (M). We show that the tangent bundle TX of a k-symplectic manifold is a Clifford module over a Clifford algebra Cl(k − 1). Then an absolutely trianalytic torus in a hyperkähler manifold M with b 2 (M) 2r + 1 is at least 2 r−1 -dimensional.