Summary—Using antisera (α‐R and α‐C7Ag) directed against the conserved Gly‐Gly‐Met‐Pro‐epitope of the hsp70 family, a single antigen was identified in the humanBabesia divergensRouen 1987 isolate by Western immunoblotting and immunoprecipitation experiments. ThisB divergenshsp70 is highly conserved as shown by the analysis of five other geographicalB divergensisolates from different hosts (human and bovine). Indirect immunofluorescence assay performed on the asexual intraerythrocytic stages showed that the hsp70 is mainly cytoplasmic and stage‐independent. Heat‐shock experiments, with 20 min incubation at 40°C followed by a 10 to 50 min shift to 37°C in the presence of [35S]‐methionine, led to an increase of two hsp of 85 and 70 kDa while protein synthesis in general decreased within 10 min. Immunoprecipitations of [35S]‐methionine radiolabelled proteins with human, ox and gerbil antisera raised against variousB divergensisolates, showed the presence of aB divergens70 kDa protein which was demonstrated to be a hsp70 by coupling immunoblotting assays with α‐C7Ag serum on the same immunoprecipitated material. During human babesiosis, theB divergenshsp70 appears as an early antigen during the acute phase. These results are in agreement with the use of theB divergenshsp70 as an essential valence antigen i