ABSTRACT We examine the early evolution of a neutron star with a very strong magnetic ®eld(B$ 4´10 13 G) that occupies a signi®cant fraction of the core volume. The electricalconductivity of the core matter is a strong function of the magnetic ®eld, therefore theevolutionofmagnetizedneutronstars(magnetars)maywellbedifferentfromthatofordinaryradiopulsars. We consider magnetohydrodynamic processes in the core for two possiblemodels of nuclear matter, with normal and super¯uid neutrons. In the case of the normalmatter,anenhancementoftheresistivityperpendiculartothemagnetic®eldcanresultinrapid®eld decay during the early evolutionary stage. If neutrons are in the super¯uid state, we ®ndthat the Hall effect can lead to oscillatory behaviour of the magnetic ®eld. This oscillatorybehaviouris caused bythegeneration oflarge-scale helicoid modes resulting from non-linearcoupling between the different ®eld components.Key words: MHD ± stars: magnetic ®elds ± stars: neutron ± pulsars: general. 1 INTRODUCTIONThe magnetic ®elds of relatively young pulsars, as inferred fromtheirspin-downrates,lieinanarrowrange4´ 10