The progressive accumulation of extracellular amyloid plaques in the brain is a common hallmark of Alzheimer's disease (AD). We recently identified a novel species of Aβ phosphorylated at serine residue 8 with increased propensity to form toxic aggregates as compared to non-phosphorylated species. The age-dependent analysis of Aβ depositions using novel monoclonal phosphorylation-state specific antibodies revealed that phosphorylated Aβ variants accumulate first inside of neurons in a mouse model of AD already at 2 month of age. At higher ages, phosphorylated Aβ is also abundantly detected in extracellular plaques. Besides a large overlap in the spatiotemporal deposition of phosphorylated and non-phosphorylated Aβ species, fractionized extraction of Aβ from brains revealed an increased accumulation of phosphorylated Aβ in oligomeric assemblies as compared to non-phosphorylated Aβ in vivo. Thus, phosphorylated Aβ could represent an important species in the formation and stabilization of neurotoxic aggregates, and might be targeted for AD therapy and diagnosis.