Aim: To evaluate the effect of water activity (a(w) 0.98-0.89, adjusted with glycerol, sorbitol, glucose, or NaCl) and temperature (5-25 degrees C) on the lag phase and radial growth rate (mm day(-1)) of the important citrus spoilage fungi, such as Penicillium italicum and Penicillium digitatum grown in potato dextrose agar (PDA) medium. To select, among models based on the use of different solutes, a model fitting accurately the growth of these species in relation to a(w) and temperature. Methods and Results: Extensive data analyses showed for both Penicillium species a highly significant effect of a(w), temperature, solutes and their interactions on radial growth rate (P < 0.0001). Radial growth rate was inhibited and the lag phase (i.e. the time required for growth) lengthened as the a(w) of the medium decreased. NaCl appeared to causes the greatest stress on growth when compared with other nonionic solutes. Penicillium italicum stopped growing at 0.96 a(w) and P. digitatum at 0.93 a(w). Under the dry conditions where growth was observed, P. italicum grew faster than P. digitatum at low temperature and P. digitatum remained more active at ambient temperature. Multiple regression analysis applied to the square roots of the growth rates observed in the presence of each solute showed that both the 'glycerol model' and the 'sorbitol model' yielded a good prediction of P. italicum growth and the 'sorbitol model' gave an accurate fit for P. digitatum growth, offering high-quality prediction within the experimental limits described. Conclusions: Mathematical models describing and predicting, as a function of a(w) and temperature, the square root of the radial growth rate of the agents responsible for blue and green decays are important tools for understanding the behaviour of these fungi under natural conditions and for predicting citrus fruit spoilage. Significance amd Impact of the Study: Implementation of these results should contribute towards a more rational control strategy against citrus spoilage fungi.