Lactic and butyric acid production by bacterial flocs in a continuous culture obeyed different physiological constraints. The butyric acid rate of production was constant and independent of the growth rate [0.012 +/- 0.001 gBUT/(L.h)], whereas lactic fermentation occurred only beyond a critical growth rate (0.25 +/- 0.05 h(-1)) and was apparently associated with an abrupt drop in biomass. Principles of modeling used to describe a Crabtree effect in Saccharomyces cerevisiae were found to apply to lactic acid production by flocs. A rank of "physiological unit" (or "metabolic unit") can be attributed to the bacterial floc. From a practical point of view, the production of fermentation products by stable flocs, naturally resistant to contamination, opens the possibility of industrial production by continuous cultivation by using flocs-forming consortia.