Aims: This study compares the effect of temperature (4-37degreesC) and water activity (a (w) : 0.99-0.87) and their interactions on the germination rates, lag times prior to germination and mycelial growth 'in vitro ' of Penicillium digitatum , P. italicum and Geotrichum candidum , the main postharvest pathogens affecting citrus fruits. Methods and Results: Germination and growth were markedly influenced by temperature and a (w) . Generally, lag times were longer and germination and growth rates were slower when conditions of temperature and a (w) were far from optimum. All the studied species were able to germinate over a range of 4-30degreesC at 0.995 a (w) , although in non-optimal conditions P. digitatum only reached 40-60% of germinated conidia. At low temperatures, P. italicum germinated and grew faster than P. digitatum and G. candidum , particularly at 0.95 a (w) . Penicillium italicum was also able to germinate and grow in the driest studied conditions (0.87 a (w) ), while G. candidum did not germinate under 0.95 a (w) . Conclusions: Knowledge of the ecological requirements of these fungi is important in order to understand their behaviour in natural situations and to predict fungal spoilage on citrus fruits.