Urea-extractable proteins have been purified from the cephalothoracic cuticle of mature Araneus diadematus spiders. Two-dimensional gel electrophoresis showed at least 12 major proteins, with pIs between 4.5 and 8.5. Five proteins were purified and their primary structure determined, using a combination of mass spectrometry and Edman degradation. Based on the amino acid sequence the proteins can be divided into two groups, both characterized by hydrophobic regions dominated by Ala, Pro and Val. Sequence similarity was observed between all the spider cuticle proteins and a number of proteins from other arthropod cuticles. Although the similarity seemed to be confined only to a region in the central part of the molecules, it does link these very distantly related species.