The amidase from a wild-type strain 8602 of Pseudomonas aeruginosa was altered by site-directed mutagenesis at position 138 by replacing Trp for Gly or Ser. The mutations introduced in the amidase gene by site-directed mutagenesis were confirmed by sequencing the cloned PCR-amplified altered genes. The recombinant wild-type and altered (W138G and W138S) enzymes were purified by affinity chromatography followed by gel filtration chromatography with a final recovery of enzyme activity in the range of 60-80%. The purified enzyme preparations were apparently homogeneous on SDS-PAGE with a Mr of 38.000 dalton as well as on native PAGE with Mr of 150.000 dalton. Both altered amidases revealed several differences in kinetic properties and stability compared with the wild-type enzyme namely in substrate specificity, sensitivity to urea, optimum pH, heat and pH stabilities. The W138G enzyme acted on acetamide, acrylamide, phenylacetamide and p-nitrophenylacetamide whereas the W138S amidase acted only on acetamide, acrylamide and exhibited a very low activity on p-nitrophenylacetamide. The kinetic constants revealed that the W138G enzyme exhibited a higher affinity for acrylamide and acetamide than the W138S enzyme. Both enzymes exhibited optimum pH for activity in the range of 8 - 9 whereas they were both resistant to urea inhibition. The heat stability of both enzymes revealed that they were less thermostable than the wild-type enzyme since the mutant (W138S and W138G) enzymes exhibited t1/2 values of 3.5 and 7.0 min. at 55ºC ,respectively. Therefore, the substitution W138G/W138S on the amidase molecule produced mutant enzymes which exhibited resistance to urea inhibition, altered substrate specificity and thermolabile compared with the wild-type enzyme.