1. A small protein of M(r) 10 kDa has been isolated by reverse-phase chromatography of the basic proteins contained in the coelomic fluid and cell lysate of the earthworm Eisenia fetida andrei. 2. The protein crossreacted in dot-blot with an anti-bovine ubiquitin antiserum. 3. Its N-terminal primary structure was determined by automatic Edman degradation on 26 consecutive amino acids and showed 69% (based on the 26 amino acids) or 82% (based on the first 19 consecutive amino acids) identity with many ubiquitins and similar charge and hydrophobicity profiles and secondary structure conformation.