A magnetic field is an inescapable environmental factor for plants in the soil. However, its impact on plant growth is not well understood. In order to learn how magnetic fields affect plants, the effects of alternating the magnetic field (MF) on the yield and ion accumulation in the leaves of strawberry ( Fragaria x ananassa ) was studied. Short day strawberry cv. Camarosa plants were treated with magnetic field (MF) strengths of 0.096, 0.192 and 0.384 Tesla (T) in heated greenhouse conditions. Fruit yield and fruit number per plant, and average fruit weight were higher at low MF strength than control and high MF strength. Increasing MF strength from control to 0.096 T increased fruit yield per plant (208.50 and 246.07 g, respectively) and fruit number per plant (25.9 and 27.6, respectively), but higher MF strengths than 0.096 T reduced fruit yield and fruit number. All of the MF strengths increased average fruit weight as compared with the control, although the largest fruit weight (8.92 g) was obtained at 0.096 T strength. Increasing MF strength from control to 0.384 T increased contents of N, K, Ca, Mg, Cu, Fe, Mn, Na and Zn, but reduced P and S content. © 2004 Taylor & Francis.