Fungal colony size development, often measured as the increase in colony diameter, is used frequently as a parameter for presenting and modelling fungi's biomass growth. Another measure of fungal growth is the heat production rate (thermal power), which represents the metabolic activities of the fungi. In this study, the colony size and the heat production rate were simultaneously measured on a filamentous fungus, Penicillium brevicompactum, at five temperatures. The colony growth was recorded by digital photography and the colony size was quantified by image analysis. The heat production rate was measured by an isothermal calorimeter. The results showed that the growth of mould is temperature dependent. During the active growing stage, the colony size increase is correlated to the heat produced by the mould.