Bees and wasps were studied on 23 south-exposed sites in chestnut forests on the southern slopes of the Swiss Alps. Window interception traps, each combined with a yellow water pan, were operated from March to September in 1997. Of the 301 identified species (from 51.641 individuals), four species are new for Switzerland and 40 species new for the Ticino County. In spite of the investigations done in the past and more recently, these new species show how little the fauna of bees and wasps is known in southern Switzerland. On the other hand, different methodological approaches (standardised traps during a long period of time instead of catching with sweep-nets during short periods) could explain the amount of additional species. Moreover, chestnut forests and burnt stands are rather uncommon collecting grounds for hymenopterologists. Nevertheless, burnt and coppice sites seem to be important habitats for a large number of bees and wasps. Since the surface area and density of the chestnut forests in southern Switzerland have increased considerably in the last 50 years due to the abandonment of traditional rural and silvicultural activities, the openings created by fire may have become essential refuge habitats for some of the bee and wasp species.