The mechanism of salt tolerance in the extremely halotolerant bacterium Micrococcus sp. Was investigated. Two cured Micrococcus strains, GME (lacking pGB 1, pGB2 & pGB3 Plasmids) & GMA (lacking pGBl & pGB2 Plasmids) were used. Both of them were sensitive to high NaCl concentration & unable to grow to 20% NaCl. Both strains were grown on minimal media containing different concentration of methionine or proline or glycine or glycerol. It was found that the presence of either methionine or proline or glycine had increase the salt tolerance of strain GMA but not GME. It was conluded that these amino acids are probably acts as compatible solutes to increase the salt tolerance, or the methionine & glycine acting as a precursor for the synthesis of the well-known compatible solute the glycine betaine. These amino acids did not increase the salt tolerance of strain GME probably because this strain lacking the plasmid pGB3 (63.095 kbp) which might carry genes responsible for transporting these amino acids into the cell or probably carrying genes important for synthesizing betaine from its precursor, the methionine or glycine. In addition to studying the effect of amino acids as a compatible solutes, the internal ion concentration of Micrococcus strain Gl (W.T.) growing in the different concentration of NaCl was determined using the atomic absorption flame emission spectrophotometry. It was found that strain Gl accumulate Na & Mg ions as osmotic stabilizer when grown at high NaCl concentration but did not accumulate K+ or Ca+2 ions. According to these results, it seems that this halotolerant bacterium use a unique mechanism of salt tolerance differ than any other mechanism known. This mechanism include the concentration of the inorgamic Na+ & Mg ions as osmotic stabilizer in addition to the organic glycine, methionine & proline as a compatible solutes. Such a unique complicated mechanism might explain how a bacterial cells such as strain G1 can grow in NaCl concentration raging from 0% to the saturation level 32% NaCl. The susceptibility of strain Gl (W.T.) & the two cured strains GMA & GME to heavy metals, mercury & cadmium, was investigated. All strains were sensitive to mercury. However they showed different sensitivity to cadmium. Strain Gl & GMA were resistant and were able to grow in media containing 20ug/ml, while strain GME was sensitive & failed to grow on media containing ug/ml. It was concluded that the plasmid pGB3 might play a role in the'resistance to cadmium, since this plasmid is absent in the cadmium sensitive strain GME & present in the cadmium resistant strain Gl&GMA. Susceptibility of strain Gl to different antibiotics were tested on YMC media containing 2% or 10% NaCl. While th susceptibility of strain GMA & GME to antibiotics were tested on YMC media containing 2% NaCl strain Gl was resistant to all antibiotics when tested on media containing 10% NaCl. However it was, (like strain GMA & GME) very sensitive to all antibiotics on media containing 2% NaCl. The following was concluded: First: the presence of high NaCl concentration in the media heavily effected the activity of the antibiotics or induce the bacterial cell by some mechanism to be resistant to antibiotics Second: plasmids of strain Gl play no role in antibiotic resistance, since both strains (the wild type & the Cured), showed the same susceptibility pattern. Factor affecting transformation of intact or protoplasts the halotolerant Micrococcus strain with chromosomal or plasmid DNA were studied. The following result were obtained. First: the intact or protoplast cells of the halotolerant Micrococcus sp. Were untransformable with plasmid DNA Second: intact cells of strain Gl were untransformable with chromosomal DNA Third: transformation frequencies of protoplast cells with chromosomal DNA were higher when the overlay method was used as a plating method compared with the direct spreading method. Fourth the transformation frequency with chromosomal DNA was greatly affected by lysozyme concentration used for protoplast formation. Transformation frequency was higher when cells were treated with the 20ug/ml lysozyme for 10-12 min., compared with 2u,g/ml or 1000u.g/ml under the same conditions.