βγ‐crystallins are one major component of vertebrate lenses. Here the isolation and characterization of a cDNA, coding for the first βγ‐crystallin molecule from an invertebrate species, the marine sponge Geodia cydonium, is described. The size of the transcript as determined by Northern blotting was 0.7 kb in length. The deduced amino acid sequence consists of 163 aa residues and comprises four repeated motifs which compose the two domains of the βγ‐crystallin. Motif 3 contains the characteristic βγ‐crystallin ‘Greek key’ motif signature, while in each of the three other repeats, one aa residue is replaced by an aa with the same physico‐chemical property. The sponge peptide shows striking similarities to vertebrateβγ‐crystallins. Analysis by neighbour joining of the sponge motifs with the two motifs present in spherulin 3a of Physarum polycephalum shows that motif 4 of the sponge βγ‐crystallin was added as the last single sequence to the tree. The data support the view that the βγ‐crystallin superfamily, present in eukaryotes, evolved from a common ancestor including also the sponge βγ‐crystallin.