[en] This research work had an objective to improve bacterial activity in degrading a herbicide: 2,4-dichlorophenoxyacetic acid (2,4-D). The bacterial isolate 3_A, kept in the culture collection of Khon Kaen University that could degrade 2,4-D, was employed in this experiment. Cell suspension of isolate 3_A was exposed to gamma irradiation at various doses (1-5 kGy). The isolated survivors were screened on the basis of forming larger colonies than the parent strain 3_A when grown on mineral salts agar containing 2,4-D (MS⁺2,4-D) as the sole carbon source. We obtained 70 effective isolates which 6 isolates called 3_AI2-21, 3_AI2-23, 3_AI1-51, 3_AI2-71, 3_AI1-52 and 3_AI2-73 were chosen for further studies. These 6 irradiated isolates together with the parent strain were characterized using morphological, physiological and biochemical tests. They were all identified as Pseudomonas cepacia. All isolates had optimal growth pH of 7 and grew best at 30^oC. Biodegradation experiments performed in mineral salts medium containing 200 ppm of 2,4-D showed that after 20 days of incubation 36.9%, 65.3%, 57.2%, 54.8%, 53.4%, 47.3% and 45.8% of 2,4- D was degraded by isolates 3_A, 3_AI2-21, 3_AI2-23, 3_AI1-51, 3_AI2-71, 3_AI1-52 and 3_AI2-73, respectively. Comparing the irradiated strains with parent strain 3_A revealed that the isolate 3_AI2-21 was the most effective one as it could degrade 2,4-D about 28.4% greater than the parent strain 3_A.