Heat shock protein 90 (Hsp90) is 1 of the most abundant and evolutionarily conserved proteins. In most species, Hsp90 is essential for proper cell function. In this study, we present the molecular analysis of Hsp90 from Eimeria species, the causative agents of avian coccidiosis. The full-length Eimeria acervulina Hsp90 complementary DNA was isolated from intestinal intraepithelial lymphocytes of Eimeria-infected chickens. From evolutionary analysis and sequence identity, it is likely that Eimeria Hsp90 sequences described thus far encode the cytosolic versions of the protein. Although at the nucleotide and amino acid levels Eimeria tenella and E. acervulina Hsp90 are highly similar, their expression profiles differ considerably. Although E. tenella transcripts were detected in all developmental stages tested, E. acervulina transcripts were not found in oocysts undergoing sporulation or in fully sporulated oocysts, suggesting that messenger RNA expression may be regulated quite differently between Eimeria species.