The sequence of the entire chick 90 kDa heat shock protein (hsp90), the non hormone binding component of the heterooligomeric form of steroid receptors, is reported. A comparison of the amino acid sequence of the chick hsp90 to that of the homologous hsp90 from yeast to man, reveals 64–96% identity respectively, and even with E. coli hsp90 an identity of 44% is observed. Analysis of the sequence and a secondary structure prediction of chick hsp90 suggest that two hydrophilic regions A and B, predicted in α-helix may play a role in the interaction of hsp90 with other proteins such as steroid hormone receptors. While there are regions of the sequences completely conserved in all hsps90, the most negatively charged hydrophilic region (A) is absent in the E. coli protein.