Fusarium verticillioides is recognized as the main maize pathogen in the world, causing ear rot. Colonized kernels are commonly contaminated by fumonisins B (FBs), secondary metabolites toxic and carcinogenic to animals and humans. In this work we studied how changing temperature (T=20,25,30 C) and water activity (aw=0.90, 0.95, 0.99) affects both FBs production and the expression of FUM2 and FUM21 genes in two F. o verticillioides strains (ITEM10027 and ITEM1744) grown 21 d in FB-inducing (Malt Extract Agar) and FB-inhibiting (Czapeck Yeast Agar) liquid media. Gene expression analysis, carried through RealTime (RT)-PCR, and FBs quantification with HPLC, were conducted. Fumonisins were produced by both strains, and ITEM10027 was proved the highest producer (10450 vs 2438 ppb). FUM2 and FUM21 were expressed in all studied conditions, with the former around 10 times more expressed than the latter. The peak of expression was observed after 14 and 21 days of incubation respectively for FUM2 and FUM 21. The expression of both FUM genes analyzed was more influenced by temperature than by aw in the studied range. Changes in temperature produced limited variation for FUM2 expression, while the highest expression for FUM21 was observed at 25oC. Moreover it was proven that incubation time of fungal pure cultures significantly influenced the two FUM genes expression and FBs production level, which resulted straight correlated. Work supported by the Italian Ministry of University and Research, PRIN 2007.