Fusarium verticillioides and F proliferatum isolates were inoculated in mixed cultures with Aspergillus parasiticus on irradiated maize grain at two different inoculum concentrations (2 Â 10 5 and 2 Â 10 2 conidia g À1 dry maize). The treatments were 0.93±0.98 water activity (a w ) and 15 and 25°C for 28 days. A complex relationship was found between a w , temperature, inoculum concentration and the interactions which took place between fumonisin and a¯atoxin producers. In general, A parasiticus reduced F verticillioides and F proliferatum populations (by 6±36%) but did not affect fumonisin B 1 production by these species. In contrast, while the Fusarium species were not able to decrease A parasiticus populations, they signi®cantly reduced a¯atoxin B 1 accumulation (by 30±93%).