We study standard Johnson differential V photometry of the RS CVn binary \object BM CVn spanning over a quarter of a century . Our main aims are to determine the activity cycles , the rate of surface differential rotation and the rotation period of the active longitudes of \object BM CVn . The Continuous Period Search ( CPS ) is applied to the photometry . The changes of the mean and amplitude of the light curves are used to search for activity cycles . The rotation period changes give an estimate of the rate of surface differential rotation . The Kuiper method is applied to the epochs of the primary and secondary minima to search for active longitudes . The photometry reveals the presence of a stable mean light curve ( MLC ) connected to the orbital period P _ { \mathrm { orb } } = 20 \fd 6252 of this binary . We remove this MLC from the original V magnitudes which gives us the corrected V ^ { \prime } magnitudes . These two samples of V and V ^ { \prime } data are analysed separately with CPS . The fraction of unreliable CPS models decreases when the MLC is removed . The same significant activity cycle of approximately 12.5 years is detected in both V and V ^ { \prime } samples . The estimate for the surface differential rotation coefficient , k \geq 0.10 , is the same for both samples , but the number of unrealistic period estimates decreases after removing the MLC . The same active longitude period of P _ { \mathrm { al } } = 20 \fd 511 \pm 0 \fd 005 is detected in the V and V ^ { \prime } magnitudes . This long–term regularity in the epochs of primary and secondary minima of the light curves is not caused by the MLC . On the contrary , the MLC hampers the detection of active longitudes .