A cDNA encoding vitellogenin (VTG), a precursor of a major yolk protein, vitellin (VTN), was isolated from cladoceran crustacean Daphnia magna. The deduced amino acid sequence of DmagVTG1, the polypeptide encoded by the cDNA, contained a possible signal peptide sequence of 16 amino acid (aa) residues. The possible mature form of DmagVTG1 consists of 1985 aa residues with a calculated molecular mass of 223,070 Da. The large lipid transfer (LLT) module and a part of the von Willebrand factor D (VWD) module found in the aa sequences of VTGs of many other organisms are well conserved in DmagVTG1. Phylogenetic analysis suggested that the LLT module of DmagVTG1 is more closely related to those of insect VTGs than those of decapodan crustaceans. A unique feature of DmagVTG1 is that it has a superoxide dismutase (SOD)-like domain at its NH(2)-terminus. Antisera against the SOD-like domain, the NH(2)-terminal part of the VTG domain and the COOH-terminal part of the VTG domain, respectively, were prepared and used for analysis of D. magna yolk proteins. Six species (I to VI) of major protein complexes were found in D. magna parthenogenetic eggs isolated immediately after ovulation. Complexes IV and V were the most abundant. DmagVTG1 was a component of Complexes III, IV and V, and the most abundant polypeptide in D. magna eggs. The protein complexes underwent gradual proteolysis during development. One of the primary sites of cleavage was between the two successive Arg residues located at the 1454th and 1455th positions of DmagVTG1.