Let X be a countable discrete abelian group with automorphism group Aut(X ). Let ξ 1 and ξ 2 be independent X -valued random variables with distributions µ 1 and µ 2 , respectively. Suppose that α 1 , α 2 , β 1 , β 2 ∈ Aut(X ) andAssuming that the conditional distribution of the linear form L 2 given L 1 is symmetric, where L 2 = β 1 ξ 1 + β 2 ξ 2 and L 1 = α 1 ξ 1 + α 2 ξ 2 , we describe all possibilities for the µ j . This is a group-theoretic analogue of Heyde's characterization of Gaussian distributions on the real line.2000 Mathematics subject classification: primary 60B15; secondary 62E10.