We show that there exist mixing zero-entropy algebraic $\mathbb
{Z}\sp 8$-actions that are measurably and topologically conjugate but
not algebraically conjugate. This result gives the first known
examples of mixing zero-entropy algebraic $\mathbb {Z}\sp d$-actions
that do not have rigidity properties, and it provides a negative
answer to the isomorphism rigidity problem raised in B. Kitchens and
K. Schmidt (Isomorphism rigidity of irreducible algebraic $\mathbb
{Z}\sp d$-actions, Invent. Math. 142 (2000), 559‒577)
and Schmidt ("The dynamics of algebraic $\mathbb {Z}\sp d$-actions" in
European Congress of Mathematics (Barcelona, 2000), Vol. 1,
Progr. Math. 201, Birkhäuser, Basel, 2001,
543‒553).