Let .M; I; !; / be a nearly Kähler 6-manifold, that is, an SU.3/-manifold with .3; 0/-form and Hermitian form ! which satisfiesfor a nonzero real constant . We develop an analogue of the Kähler relations on M , proving several useful identities for various intrinsic Laplacians on M . When M is compact, these identities give powerful results about cohomology of M . We show that harmonic forms on M admit a Hodge decomposition, and prove that