The contemporary energy reduction agenda for housing is leading to improved levels of air-tightness and increasing adoption of Mechanical Ventilation with Heat Recovery (MVHR). The effect of these measures on indoor microbiology remains significantly under-researched and may have important consequences on the concentrations and diversity of airborne fungi and bacteria. This study investigated indoor microbiology in six contemporary airtight dwellings. Concentrations of fungi and bacteria were measured during the winter season. Mean bedroom fungi concentrations of 18.4 CFU/m3 and bacteria concentrations of 212.6 CFU/m3 were recorded. Concentrations of fungi ranged from 3-59 CFU/m3 and bacteria from 19-607 CFU/m3. It is suggested that the filtration of air through the MVHR system may have limited the migration indoors of fungi of outdoor origin, explaining the low bedroom concentrations. The findings demonstrate the need for further research to identify the effect of ventilation strategies on fungal and bacterial concentrations indoors, particularly in airtight dwellings.