Poly-gamma-glutamic acid (gamma-PGA) is a retaining agent; it has applications in the food, medicine, agriculture, cosmetics and wastewater treatment industries. Most of the gamma-PGA producing strains belong to the genus Bacillus. This study reports on a novel gamma-PGA producing species. Bacillus siamensis SB1001 was screened and isolated from organically cultivated soybeans exhibiting a high gamma-PGA producing ability. The fermentation medium and culture parameters for gamma-PGA production by Bacillus siamensis SB1001 were optimized by statistical methods. The sucrose, L-glutamic acid and dipotassium phosphate in the medium were shown to be the significant factors of the gamma-PGA production, and the optimum medium obtained consisted of the following: 106.86 g/L sucrose, 69.84 g/L L-glutamic acid and 2.39 g/L dipotassium phosphate. Using the optimized medium, 25.22 g/L gamma-PGA were produced with a productivity of 1.05 g/L/h. The gamma-PGA obtained had a molecular weight of 7.9 x 10(5) Da and a polydispersity index of 2.34, and the ratio of D-/L-glutamic acid was 89.71%:10.29%. To the best of our knowledge, this is the first report of gamma-PGA production by B. siamensis strain. B. siamensis SB1001 has great potential as an industrial gamma-PGA producer.