Because sponges (Porifera) are generally the dominant group of multicellular animals found in the marine benthos they are key animals for the testing of biomarkers in this biotope. Previously, it was shown that the heat shock protein hsp70 from the marine sponge Geodia cydonium is a useful biomarker for following biological responses to selected physical and chemical stressors. One protein which is critical for hsp70 function in other organisms is the DnaJ-like protein. In this study we therefore isolated and characterized a cDNA encoding the DnaJ-like protein from G, cydonium designated GCDNAJ. GCDNAJ is 1476 base pairs (bp) long with a 1236 bp open reading frame encoding a protein of 45 542 daltons. Northern blot analysis revealed a transcript size of 1.5 kilobase (kb). The deduced amino acid (aa) sequence of GCDNAJ contains motifs characteristic of most prokaryotic and eukaryotic DnaJ family members. In response to a 7 degrees C temperature increase, expression of the transcripts encoding hsp70 and DnaJ-like protein increased as much as 18-fold and 3-fold, respectively, within a 8 h period. From these data we conclude that the levels of both hsp70 and DnaJ-like protein are likely to be important to the adaptation to temperature increases in G. cydonium.