Abstract: Intracellular glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) from Bacillus schlegelii, Bacillus sphaericus, Bacillus marinus and Bacillus circulans were variably sensitive to growth conditions (temperature, pH and NaCl) and so were G6PDH and 6PGDH within the same strain. Thermostability of 6PGDH and G6PDH vary among the studied species; the difference in thermostability between the more stable G6PDH and the less stable 6PGDH from the same species may compensates the lower G6PDH and the higher 6PGDH activity found in this study. These results may indicate the existence of separate regulatory pathways for the two enzyme activities; one pathway may be related to thermostability and other pathways may be related to other growth conditions of pH and NaCl.