A hexameric protein of high (18.8%) aromatic amino acid content has been isolated from the haemolymph of pupae of Hyalophora cecropia. It is the major protein present in the haemolymph, and also occurs in the fat body. It is distinct in its behaviour on polyacrylamide gels and in its antigenic reactivity from the two storage proteins of H. cecropia described earlier by Tojo et al. (1978). The new protein is related to similar proteins reported earlier from several species, all of which share the common property of an unusually high content of aromatic amino acid. We propose the generic name arylphorin for such proteins. Hyalophora arylphorin is composed of six seemingly identical subunits of apparent mol.wt. 73,000. Cross-linking studies confirm the hexameric structure, and pore limiting gradient gel electrophoresis of the native protein is consistent with a mol.wt of about 400,000. It contains about 4% carbohydrate, consisting of glucosamine and mannose in a 1:5 molar ratio. The carbohydrate could not be removed by treatment with endoglycosidase H. Hyalophora and Manduca arylphorins are antigenically similar, but their antibodies did not precipitate Calliphora arylphorin. The concentration of Hyalophora arylphorin in haemolymph was examined during the last larval and the pupal-adult moults. In both cases a 95–100% drop in concentration occurred between apolysis and ecdysis.