The aims of this study were to evaluate the effects of water activity, temperature, incubation time and their interactions on lag phase, growth rate and ochratoxin A (OTA) production by strains belonging to the Aspergillus niger aggregate on irradiated peanut seeds. In the temperature and water activity range assayed, the optimal conditions of growth for RCP42 and RCP176 strains on irradiated peanut seeds were 0.995 a(w) and 30 degrees C being the growth rates of 12.4 and 14.6 mm/day, respectively. The maximum OTA production occurred at 0.973 a(w) and 25 degrees C for both strains assayed; whereas the minimum OTA production was obtained at 0.951 a(w) and 15 degrees C, at 14 and 21 days of incubation for RCP42 and RCP176 strains, respectively. The amount of OTA accumulated during 21 days assayed by both strains varied from 6.5 to 460 microg/g and from 10 to 210 microg/g with mean levels of 119.2 and 97.5 microg/g for RCP42 and RCP176 strains, respectively. The variance analysis (ANOVA) revealed that OTA concentration produced by RCP42 strain was significantly (p<0.0001) greater than that produced by RCP176 strain. If the strains with which the experiments were carried out were representative of the Aspergillus niger aggregate toxigenic species and the water activity in peanut seeds stored at 0.910 or lower was maintained, OTA production would be reduced during at least 21 days at variable temperatures.