Leucine-rich repeats (LRRs) are 22-28 amino acid-long sequence motifs found in a family of cytoplasmic, membrane and extracellular proteins. There is evidence that LRRs function in signal transduction, cellular adhesion and protein-protein interactions. Here we report unusual properties of a synthetic LRR peptide derived from the sequence of the Drosophila membrane receptor Toll. In neutral solution the peptide forms a gel revealed by electron microscopy to consist of extended filaments approximately 8 nm in thickness. As the gel forms, the circular dichroism spectrum of the peptide solution changes from one characteristic of random coil to one associated with beta-sheet structures. Molecular modelling suggests that the peptide form an amphipathic structure with a predominantly apolar and charged surface. Based on these results, models for the gross structure of the peptides filaments and a possible molecular mechanism for cellular adhesion are proposed.