Synthesis and excretion of Bacillus stearothermophilus cl-amylase is analyzed in fed-batch cultivations of Escherichia coli JM103[pMK79] and E. coli Jm103[pMK57], the former strain containing the plasmid-encoded Vitreoscilla hemoglobin (VHb) gene (vgb) and the latter strain being devoid of this gene. Fed-batch operation is observed to be substantially superior to batch operation as concerns the alpha-amylase production rate and the extent of excretion of the enzyme. Faster feeding of a nutrient medium (LB or M9) discourages synthesis of a-amylase. While synthesis of cl-amylase in the vgb(-)strain is discouraged when oxygen availability is reduced, the reverse is the case with the vgb(+) strain, the promotion of cl-amylase synthesis in the latter strain being linked to the synthesis of VHb. Increased availability of the principal carbon source (glucose) in a defined medium leads to overproduction of both alpha-amylase and VHb under oxygen limitation, which may be responsible for the segregational instability observed with the vgb+ strain. The very high extents of excretion of a-amylase attained in fed-batch cultures are encouraging for downstream processing of the recombinant protein.