We report on the electron-spin-echo decay of triplet [2-H-2]-dodecapentaenal in stretched polyethylene films. Deuterium nuclear-spin-induced modulations have been observed and analyzed, and a value of 0.12 +/- 0.01 is found for the spin density on the carbon atom at position 2, The agreement of this value with that calculated by semiempirical molecular-orbital methods provides the basis for a discussion of the spin-density distribution along the conjugated carbon-carbon chain of polyenes in the lowest triplet state.