Contamination of pistachio nut by aflatoxin is one of the most important problems, which make a lot of difficulties in production, consumption and exports.

In respect to the importance of the matter, the population density of Aspergillus flavus and Aspergillus niger groups were examined for the following items through years 1998 - 2001: - Nuts which their hulls were cracked - Nuts which were fallen on the soil under the trees during the growing season - Nuts located one meter above the soil surface in four geographical directions in the trees - Plant debris - Soil - Different types of manures customized to be used in the orchards - Processing terminals ( 17 plants ) and processed nuts - Density fluctuations of airborne spores in orchard spaces The results revealed that population density of A. flavus and A. niger group has an increasing trend from early spring (March) to late summer (September) which is also its maximum, but it will decrease during next seasons (November to February). The population density of spores of A. flavus and A. niger groups has a slight fluctuation in the fall and winter seasons, which differ with time. The populations density of spores in the space of under studied orchards decrease respectively from Rafsanjan, Sirjan, Zarand and Kerman. Density of spores in the space of different terminals during the processing period was maximum in hulling and washing in respect to other processing steps. Comparison of the fluctuations trend of spore density and temperature in pistachio orchards showed a positive correlation through May to June. But increasing trend of spore density will last in September and decreased afterward. Density spore with the relative humidity and rainfall did not showed any specific trend during the study years. The population density of studied fungi in leaves was less than rubbish remaining from pistachio nuts processing. The percent of recovery of Aspergillus species in study samples was 81 percent for the rubbish and 85 percent for the leaves. The average population density of A. flavus and A. niger group in different type of manures showed descending trend as: sheep, cow and poultry manures. The result of this study show that Aspergillus is a soil-borne fungus and its population in the soil can be affected by different irrigation regimes. The population descending order of population density of fungi is shown to be as: Cracked hull nutsfallen nuts on the soilin contact with the soil twigsfour geographical direction and one meter height. Population density in different sampling time was increased near to harvest. Study of processing terminals showed that infection level is higher in traditional terminals than semi-mechanized one. The results showed that shower types has a better efficiency than basin types (steady or flowing water) to decrease population of Aspergillus species. The combination of these two mentioned types is recommended to establish and optimize system for washing and isolation of the safe and infected nuts from each other. A correlation existed between coloration of shell and infection to A. flavus and A. niger, and it can be use as a factor for separation of contaminated nuts. Population density of studied fungi was nusignificantly change in different sampling days.