Two replicative forms characterize the asexual cycle of the protozoan parasite Toxoplasma gondii: rapidly growing tachyzoites and slowly dividing encysted bradyzoites. The mechanisms that regulate the transition between these two stages are not clearly understood. However, stress inducers that also activate heat shock protein expression can trigger formation of bradyzoites in vitro. Here, we studied the association of the T.gondii Hsp90 with modulation of parasite differentiation and response to stress stimuli using RH DeltaUPRT parasites and the cystogenic strain ME49 and a clone derivative of that strain, PK. Our results show that Hsp90 transcript and protein levels increase under stress or bradyzoite differentiation conditions. Moreover, fluorescence microscopy studies revealed that Hsp90 is present in the cytosol of tachyzoites and both in the nucleus and cytosol of mature bradyzoites, suggesting a correlation between its subcellular organization and these two developmental stages. To further characterize the role for Hsp90 in bradyzoite differentiation, T.gondii tachyzoite mutants that are defective in differentiation showed the same staining pattern as tachyzoites under differentiation conditions. In addition, geldanamycin, a benzoquinone ansamycin antibiotic capable of binding and disrupting the function of Hsp90, blocked conversion both from the tachyzoite to bradyzoite and the bradyzoite to tachyzoite stage, suggesting an essential role for this protein in the regulation of stage interconversion. These results thus suggest Hsp90 may play a role in stage switch.