This article presents three interrelated theses on the mechanisms underlying developmental and transfer errors, and exemplifies these with reference to a number of English L2 developmental structures. The first thesis states that formal properties inherent to the L2 which give rise to developmental errors also set up structural predispositions for L1 transfer (Zobl 1980a). The second thesis states that, when an L1 structure conforms more closely to general acquisition regularities or processes than the L2 structure to be acquired, then the occurrence of transfer is promoted (Zobl 1980b). The third thesis, developed with reference to negation by Spanish L1 learners, states that, although the mechanisms involved in the genesis of developmental and transfer errors are the same, transfer errors may retard subsequent restructuring. The nature of the structural constellation between Spanish Li and developmental regularities in the acquisition of negation in English are examined, and a framework is proposed in which linguistic factors are seen to play a major role in protracting the restructuring of the preverbal negation rule by Spanish learners.