Stands included in seven thinning trials with Norway spruce (Piceu ahies (L.) Karst.) originally planted on highly productive sites in southern Sweden were investigated for the influence of different thinning regimes on basic density and ring width. Light thinnings from below did not have any direct effect on these properties. However, heavy thinnings that removed a total of 40% of the basal area considerably enhanced diameter development without a loss in volume production or a reduction in the uniformity of the wood. Thinning from above resulted in higher basic density and lower ring width compared with thinning from below. This was found to be an effect of selection because basic density increased with decreasing tree diameter. Diameter class had no effect on response to thinning as indicated by the fact that the smaller trees also responded strongly when thinned from above. Since no difference in basic density level between thinning regimes was found, it was concluded that differences in basic density were due to differences in annual ring width.