<jats:p>The amount of available water in the environment of micro‐organisms, defined as water activity (<jats:italic>a</jats:italic><jats:sub>W</jats:sub>), has been shown to affect growth, respiration, enzyme synthesis, sporulation and other physiological functions. The aim of this study was to examine the impact of <jats:italic>a</jats:italic><jats:sub>W</jats:sub> on production/excretion of a secondary metabolite. For this purpose, the production of β‐lactam antibiotics and biomass of <jats:italic>Streptomyces clavuligerus</jats:italic> was studied in relation to the <jats:italic>a</jats:italic><jats:sub>W</jats:sub>‐depressing agents glucose, sorbitol and NaCl. These were chosen because NaCl and sorbitol are often used to depress <jats:italic>a</jats:italic><jats:sub>W</jats:sub> and glucose was not thought to be taken up by <jats:italic>S. clavuligerus.</jats:italic> The filamentous bacterium <jats:italic>S. clavuligerus</jats:italic> NRRL 3585 (ATCC 27064) is a prokaryotic producer of penicillin N, cephalosporins including cephamycin C and clavulanic acid. Under water stress conditions, a greater effect upon antibiotic biosynthesis than upon growth was consistently observed. When <jats:italic>a</jats:italic><jats:sub>W</jats:sub> was decreased to below 0·997, antibiotic production began to decrease. For growth, inhibition was much more gradual and did not become intensive until an <jats:italic>a<jats:sub>W</jats:sub></jats:italic> of 0·990 was reached.