A putative alpha-amylase gene of Thermotoga petrophila was cloned and expressed in Escherichia coli BL21 (DE3) using pET-21a (+), as an expression vector. The growth conditions were optimized for maximal expression of the alpha-amylase using various parameters, such as pH, temperature, time of induction and addition of an inducer. The optimum temperature and pH for the maximum expression of alpha-amylase were 22 A degrees C and 7.0 pH units, respectively. Purification of the recombinant enzyme was carried out by heat treatment method, followed by ion exchange chromatography with 34.6-fold purification having specific activity of 126.31 U mg(-1) and a recovery of 56.25 %. Molecular weight of the purified alpha-amylase, 70 kDa, was determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The enzyme was stable at 100 A degrees C temperature and at pH of 7.0. The enzyme activity was increased in the presence of metal ions especially Ca+2 and decreased in the presence of EDTA indicating that the alpha-amylase was a metalloenzyme. However, addition of 1 % Tween 20, Tween 80 and beta-mercaptoethanol constrained the enzyme activity to 87, 96 and 89 %, respectively. No considerable effect of organic solvents (ethanol, methanol, isopropanol, acetone and n-butanol) was observed on enzyme activity. With soluble starch as a substrate, the enzyme activity under optimized conditions was 73.8 U mg(-1). The alpha-amylase enzyme was active to hydrolyse starch forming maltose.