As for bacteria, the percentage of Streptomyces (Table 1) increased in general with soil depth before planting as well as at harvest. It is quite revealing to compare the live weights of the desert microflora with those of more fertile soils. It has been estimated by Bollen (1959) that one hectare of fertile soil down to a depth of 17 cm (plough depth) can contain as much as 1,250 kg of bacteria and actinomycetes each and up to 2,500 kg of fungi, or a total of up to 5,000 kg of soil microorganisms. One can also estimate the live weight of desert microflora from data such as presented in Table 1. If one very conservatively assumes that for the 0-9 cm depth segment a moderately fertile soil has a live weight of only 288 kg of fungi per hectare, as cited by Alexander (1961), fungi in every gram of this soil. then one has about 35,000 Since experimentally 8,700 fungi per gram were found in NTS Area 13 soil, one can estimate a live weight of approximately 70 kg of fungi per hectare of the NTS soil before planting. Using the same basis of comparison for the 17,100 fungi per gram of NTS Area 13 soil found at harvest, the live weight would be approximately 140 kg per hectare, an increase of about 100%. Similarly, if one assumes a very conservative live weight of 175 kg of bacteria per hectare in the 0-9 cm depth segment of a moderately Fertile soil at a population count of 115 million per gram of soil (Alexander, 1961; Clark, 1967), then the 4.4 million bacteria per gram determined for the NTS soil before planting would amount to a live weight of nearly 7 kg per hectare. Using the same method for the population of 52 million bacteria per gram at harvest, one may calculate a live weight of about 79 kg of bacteria per hectare, or a 12-fold increase in the live weight of bacteria in NTS soil. The above results demonstrate that microbial activity in desert soil increases with increasing plant growth. This increase in activity is due at least in part to an increased supply of nutrients, especially root exudants. Since the common soil fungus, Aspergillus niger, can assimilate plutonium, it is reasonable to assume that other soil fungi, such as Penicillium, and probably soil bacteria, may possess similar abilities. Plutonium assimilation by soil organisms obviously requires the formation of complexes which can penetrate the microbial cell wall. Upon death of the microbial cells, the cellular contents 75