There is a growing concern of the exposure and effects that biological agents in the indoor environment can have on health of people. Recently, a report on indoor air quality and dampness and mould by the WHO (2007), concluded that sufficient epidemiological evidence is available that inhabitants of damp or mouldy buildings, both homes and public buildings, are at increased risk of experiencing respiratory symptoms, respiratory infections and exacerbations of asthma and there is clinical evidence that exposures to moulds and other dampness-related microbialagents increase the risk of rare conditions, such as hypersensitivity pneumonitis/allergic alveolitis, chronic rhinosinusitis and allergic fungal sinusitis. However, the studies on levels of bioaerosols in various residential settings are rare and sporadic. Most of these studies reported only the total counts of biological agents. But the knowledge of size distribution of bioaerosol in indoor settings is of great importance with regard to their potential health effects and to understand their fate indoors. Therefore, the present investigation was undertaken to assess the size distribution of bioaerosols in the residential houses of the South East of England.