To improve the effectiveness of research into the Siberian flying squirrel Pteromys volans, we investigated the optimal placement of nest boxes. Of the 96 boxes which we installed under various conditions at 10 sites, 47 were occupied by Siberian flying squirrels between spring and autumn. Nest-box height was positively correlated with box occupation; 90% of boxes installed at 2-2.8 m height were used. Cavity density was negatively correlated with occupation, with boxes more frequently used in forests with < 2 cavities/ha. Research in Siberian flying squirrels can thus be made more efficient if nest boxes are installed at a height of 2-2.8 m in forests with < 2 cavities/ha, and by doing observations between spring and autumn.