The presence of viable mould propagules in indoor air was investigated using the N6‐Andersen sampler in combination with DG18‐agar, in relation to house damp (characterized with a checklist) and in relation to the presence of moulds in outdoor air. The first part of the study was conducted in 46 houses in the autumn of 1987, the second part in 84 houses in May 1989. Further, in the second part, the results obtained with settlement plates (OPD) were compared with those obtained with the N6‐Andersen sampler. The number of CFU/m3in the indoor and outdoor air varied widely. A large variety of mould genera and species was isolated. Species ofCladosporium, PenicilliumandWallemiapredominated. The variability in time was high and the reproducibility of the measurements in terms of CFU/m3and of species isolated was only moderate. The low predictive value of these measurements limits their use in epidemiological studies of the relationship between exposure to moulds and respiratory symptoms. Overall, the geometric mean concentration was somewhat higher outdoors than indoors. However, the clear differences found between the number of CFU/m3belonging to different mould species in in‐ and outdoor air show that the presence of viable mould propagules in indoor air is not simply a reflection of the presence of moulds in outdoor air. The presence of moulds in indoor air was only weakly related to house damp as characterized by the checklist. High, statistically significant correlations were found between the CFU yield obtained with the OPD and the CFU/m3yield obtained with the N6‐Andersen sampler. However, the number of species isolated with the OPD was significantly lower than that obtained with the N6‐Ander