Knowledge of the ranging behavior and spatial requirements of a species is fundamental for establishing meaningful conservation strategies. Such information is lacking for the middle spotted woodpecker (Dendrocopos medius), a species endangered throughout its westpalearctic range. By radiotracking, we studied spacing behavior of this habitat specialist in a lowland oak forest of northeastern Switzerland from 1992-1996. Home range and core area size decreased from winter to late spring, with males and females having home ranges of similar size. Overlap of male home ranges was highest in winter (up to 40%) and lowest in late spring, whereas core area overlap remained low. For both home ranges and core areas, overlapping parts were used randomly in winter but more often than expected in early spring. Overlap of female ranges and of core areas did not change from early to late spring, and the shared parts of these home ranges were used as expected in both seasons. Aggressive interactions were most common in March and April and occurred mainly between individuals of the same sex. Our results suggest that the middle spotted woodpecker is not territorial in winter but defends nearly exclusive territories during spring, with both sexes participating to similar degrees in territorial defense. Based on this seasonal territoriality, we propose consideration of core areas in early spring (Mar and Apr) as a reliable estimate of the area requirements of the species to be used in management plans.