We investigated the wood ants of forest fragments composing the Sixtusbossen at Poperinge- Vleteren (Western Flanders, Belgium). Two large fragments, one with and one without wood ants, and two small forest fragments, also one with and one without wood ants (occurring), were sampled by means of pitfall traps during nine weeks in late summer and autumn 2005 with 36 traps in the large woodlands and 9 in the small woodlots. The workers of the wood ants of the Sixtusbossen are identified as Formica rufa or as the hybrid F. rufa X polyctena on the basis of the morphological criteria proposed by Seifert. Further morphological and molecular genetic work is in progress to get a clearer picture of the taxa of the Formica rufa complex and their distribution in our regions. The social organisation of the wood ants of the Sixtusbossen is as the one usually observed in the hybrid morph. Nest mounds mostly occur along the woodland edges. They vary in general appearance and are adapted to local conditions. Nests are polygynous (several or many queens per nest). New nests are formed by budding off from existing ones. Nuptial flights are short ranged and only done by males. Pairing was observed in the immediate vicinity of and on the nest mounds. Serviformica species were not found in the Sixtusbossen or nearby forests. Where such ant species occur their nests are often temporally parasitized and finally taken over by a fertilised wood ant queen after her nuptial flight. The absence of Serviformica ants implies that in our forest fragments new nest formation can only be accomplished through soil surface dispersal, hampering the (re-)colonisation of fragments where wood ants do not occur or have disappeared. Monitoring and mapping of the nest mounds during the past ten years has revealed that in the Canadabos, one of the larger fragments, the number of nests augmented from 29 in 1996 to 72 in 2006. After forest clearing during 2003 the number of nests grew from 30 to 52 in 2005. Because of tree felling during the winter 2005-2006 ...