The infectious agent in prion diseases is an aberrant-folded isoform of the cellular prion protein (PrP^C). This scrapie-related prion protein (PrP^Sc) has an increased β-sheet content, is detergent insoluble and proteinase K resistant, and accumulates in prion-infected organisms and cells. In vitro, PrP^Sc self-aggregates into amyloid fibrils. However, there is no direct experimental proof for the occurrence of PrP ^Sc-containing fibrils in vivo or in cell cultures. Applying atomic force microscopy (AFM) to scrapie-infected mouse neuroblastoma (ScN2a) cells, we discovered growing patch-like assemblies of amyloid-like fibrillar structures on the cell surfaces. Immunofluorescence and AFM images showed heterogeneous accumulation and aggregation of PrP^Sc in ScN2a cell cultures. The percentage of cells having characteristic fibrils on their surface increased with time after scrapie infection. These endogeneous fibrils had lengths from 0.5 to 3 μm and protruded from the cell surface by 108±30 nm, and thus resembled the heterogeneous shapes and networks of in vitro prepared amyloid fibrils.