We applied a kit for bioluminescent ATP measurement based on the luciferase reaction to Acidithiobacillus ferrooxidans cultures. Determination limit for monitoring cell growth in the culture was 1.1 × 106 cells per ml. Determination errors varied between 1.5 and 10 % (RSD). ATP formation correlated with cell growth and iron oxidation up to limitation by iron(II). A maximum ATP content was approximately 1 amol per cell under the growth yield on iron of 1 × 1012 cells per mol Fe(II).