Fifty hertz magnetic fields (MFs) induced the expression of heat shock proteins (HSPs) 70 and 90 in immunocytes of the mussel Mytilus galloprovincialis. Animals exposed at 300 μT for three different times (30; 2*30; 3*30 min), did not show differences in the HSP densitometric values in comparison with non-exposed mussels. At 400 μT, exposed animals showed a time-dependent increase in HSP expression as revealed by Western blot. After exposure to 600 μT, the HSP densitometric values were significantly higher than controls but not related to exposure duration. The induction of HSPs is concomitant with the activation of p38 MAP kinase signalling pathway. The present findings suggest the possibility to modulate the expression of HSPs by an appropriate time–intensity magnetic field exposure.