A Penicillium decumbens strain was collected from a water-damaged building, and the production of microbial volatile organic compounds (MVOCs) was investigated by means of headspace solid-phase microextraction, followed by GC-MS analysis. The strain was characterized by a high production of (+)-thujopsene. The influence of various temperatures, relative humidity (RH) values, substrates, and inoculum concentrations on fungal growth and (+)-thujopsene production was studied. The optimal temperature and relative humidity for P. decumbens growth were 30°C and 100% RH, respectively. In general, the more favourable the incubation parameters were for growth, the faster maximum (+)-thujopsene production was reached. Moreover, the antifungal activity of thujopsene was tested against 16 fungal strains. The growth of five of these fungal strains was negatively affected both by thujopsene alone and when grown in contact with the MVOCs produced by P. decumbens. Following these results and since growth of P. decumbens itself was also inhibited by thujopsene, an autoregulatory function for this compound was proposed. Few data are present in the literature about chemical communication between fungi. The present research could, therefore, contribute to understanding fungal metabolism and behaviour in indoor environments.