Comparatively little work has been done on particular species of fungi as habitats for insects and other invertebrates. Smith (1956) has studied Phallus impudicus (Linnaeus) Pers. and Paviour-Smith (1959, 1960a, b, 1965 and 1968) Polyporus betulinus (Bull.) Fr., while Pielou & Matthewman (1966) and Pielou & Verma (1968) have given complete lists of adult arthropods in P. betulinus and Fomes fomentarius (Linnaeus ex Fries) Kickx. from eastern Canada and Gatineau Park, Quebec, respectively. The information relating to fungi as habitats is discussed by Elton (1966). My account brings together for the first time the literature on the fauna of Ascomycetes and examines in detail the fauna of one particular ascomycete. Daldinia concentrica (Bolton ex Fries) Cesati & Notaris is an ascomycete belonging to the group Pyrenomycetes and the order Sphaeriales. Its fruiting bodies are black, carbonaceous, hemispherical stromata, sometimes reaching the size of a small apple. In Britain the fungus grows most commonly on ash (Fraxinus excelsior L.), but occasionally on beech (Fagus sylvatica L.), elm (Ulmus sp.), sweet chestnut (Castanea sativa Mill.), London plane (Platanus x hybrida Brot.) and sycamore (Acer pseudoplatanus L.). It is also found on birch (Betula pendula Roth), oak (Quercus robur L.) and gorse (Ulex europaeus L.) which have been scorched by fire. Brief references are made to its fauna in the literature. Donisthorpe (1935) names three beetles as specific to the fungus: Cryptophagus ruficornis (Stephens), Platyrhinus resinosus Scopoli and Biphyllus lunatus (Fabricius), and others including Mycetophagus atomarius Fabricius as being associated with it. Buxton (1960) gives the dipteran, Amiota alboguttata (Wahlberg) and Beirne (1952) the lepidopteran, Myelois neophanes Durrant, as being specific to this fungus. The present study attempts to assess the fungus as a habitat, describes two local variants of the fruiting body and lists over a hundred species found in about 1000 stromata examined from five different localities. It endeavours to relate the succession of animal life to the stages of development and decay of the fruiting body. Details are also given of the most important beetle species, Biphyllus lunatus. This is not intended to be a strictly quantitative study because of the problems involved in sampling.