Phosvitins are extensively phosphorylated serine-rich proteins that are derived from a large hepatic phosphoglycolipoprotein, vitellogenin, and are deposited, after suitable processing, in the eggs of oviparous vertebrates. Despite their widespread occurrence and apparent importance for early embryonic development, very few phosvitins have been sequenced thus far, including no teleost protein. We used a polymerase chain reaction (PCR)-assisted approach to isolate a vitellogenin cDNA clone, complementary to mRNA synthesized in the liver of estrogen-treated rainbow trout. This clone contains a sequence that corresponds to the composition of a phosvitin previously characterized in our laboratory. The amino terminus was identified by amino acid sequencing of the protein. The carboxyl terminus was inferred from homology with other phosvitin-specific sequences from chicken, Xenopus, and lamprey. Trout phosvitin appears to be a small, 53-residue-long protein displaying the long runs of serines that are characteristic of all phosvitins described so far.