Bacillus subtilis CBTK 106, isolated from banana wastes, produced high titres of alpha-amylase when banana fruit stalk was used as substrate in a solid-state fermentation system. The effects of initial moisture content, particle size, cooking time and temperature, pH, incubation temperature, additional nutrients, inoculum size and incubation period on the production of alpha-amylase were characterised. A maximum yield of 5345000 U mg(-1) min(-1) was recorded when pretreated banana fruit stalk (autoclaved at 121 degrees C for 60 min) was used as substrate with 70% initial moisture content, 400 mu m particle size, an initial pH of 7.0, a temperature of 35 degrees C, and additional nutrients (ammonium sulphate/sodium nitrate at 1.0%, beef extract/peptone at 0.5%, glucose/sucrose/starch/maltose at 0.1% and potassium chloride/sodium chloride at 1.0%) in the medium, with an inoculum-to-substrate ratio of 10% (v/w) for 24 h. The enzyme yield was 2.65-fold higher with banana fruit stalk medium compared to wheat bran.