It has previously been reported that amyloid-β (Aβ) peptide is neurotrophic to undifferentiated but neurotoxic to differentiated primary neurons. The underlying reasons for this differential effect is not understood. Recently, the toxicity of Aβ to neurons was shown to be dependent upon the activation of cyclin-dependent kinase 5 (Cdk5), thought to promote tau phosphorylation that leads to cytoskeletal disruption, morphological degeneration and apoptosis. Here we report that Cdk5, tau, and phosphorylated-tau (P-tau) are expressed at very low levels in undifferentiated primary neurons, but that the expression of Cdk5 and tau and the phosphorylation of tau increase markedly between 4 and 8 days of differentiation in vitro. Tau expression decreased after this time, as did the level of P-tau, to low levels by 17 days. Aβ induced tau phosphorylation of neurons only after ≥ 4 days of differentiation, a time that coincides with the onset of Aβ toxicity. Blocking tau expression (and therefore tau phosphorylation) with an antisense oligonucleotide completely blocked Aβ toxicity of differentiated primary neurons, thereby confirming that tau was essential for mediating Aβ toxicity. Our results demonstrate that differentiation-associated changes in tau and Cdk-5 modulate the toxicity of Aβ and explain the opposite responses of differentiated and undifferentiated neurons to Aβ. Our results predict that only cells containing appreciable levels of tau are susceptible to Aβ-induced toxicity and may explain why Aβ is more toxic to neurons compared with other cell types.