One of the hallmarks of Alzheimer's disease is the formation of aggregates of the tau protein, a process that can be facilitated by the presence of fibrils formed by the amyloid beta peptide (A beta). However, the mechanism that triggers tau aggregation is still a matter of debate. The effect of A beta(40) fibrils on the aggregation of the repeat domain of tau (TauRD) is investigated here by employing coarse-grained molecular dynamics simulations. The results indicate that the repeat domain of tau has a high affinity for A beta(40) fibrils, with the (261)GSTENLK(267) fragment of tau driving TauRD toward the (16)KLVFFA(21) fragment in A beta(40). Monomeric A beta(40) in which the (16)KLVFFA(21) fragment is rarely found in an extended conformation (as in the fibril), has a low affinity for the TauRD, indicating that the ability of A beta(40) fibrils to bind to the TauRD depends on the (16)KLVFFA(21) fragment of A beta adopting an extended conformation.