ABSTRACT: The distribution of aflatoxin concentration has been measured by sampling 19 process streams ranging from 1 to 1000 nuts/sample. The 19 streams comprise substantially the entire sorted product of one producer for crop year 1992. Similar results were obtained by another producer for 1993. The fraction of total aflatoxin accounted for by each process stream was computed. It was concluded that 90% of aflatoxin is contained in 4.6% of (low-quality) product; removal of this product would reduce average aflatoxin from 1.2 to 0.12 ng/g for all product sold for human consumption. On the basis of the aflatoxin distributions of the processed as well as an unprocessed stream, the conclusion is drawn that all aflatoxin found here arises in the orchard; none is produced under normal processing conditions. It is estimated that the necessary sample size (in terms of the sample mean and desired variance of the mean) for aflatoxin measurement in pistachios is given by number of nuts ) (8 -105 ng/g) mean/variance.