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The impact of supplemental food on a cyclic bank vole Clethrionomys glareolus (Schreber, 1780) population was studied at peak density. We provided high energy food (sunflower seeds) to a 3.15 ha live-trapping grid and used a 5.06 ha unfed grid as a control. Density of adult females and immatures increased 3-fold in response to the extra food. In contrast, density of adult males did not change significantly. The rise in density of adult females, but not of adult males, altered the functional sex ratio. Loss rate of red adults remained the same as for controls, whereas immature loss rate was reduced by the extra food. Dispersal of immatures fom the control into the food supplemented grid was higher than the reverse. Growth and body mass of fed immatures were lower than controls. Reduced dispersal and increased immigration of immatures both contributed significantly to the overall rise in density when food was added.