Pulse perturbation experiments performed in the steady-state of an anaerobic fluidised bed reactor, which has been operated with propionate as sole substrate for two years, have shown considerable metabolic versatility of the biofilm population. After having been fed pulsed additions of propionate or propionate plus sulfate, ethanol, formate, L(+)-lactate (in the presence and absence of propionate), and a racemic mixture of lactate (in the presence of propionate), all substrates were degraded with the exception of lactate. The propionate turnover was 71.4-75.2 mmol propionate . l(-1). d(-1) equivalent to a high biomass specific propionate degradation of 287-302 mu mol . mg protein(-1). d(-1) at low propionate concentrations during the steady state of reactor operation and 85.4 mmol propionate . l(-1). d(-1) equivalent to 343 mu mol . mg protein(-1). d(-1) at high propionate concentrations after pulsed addition of propionate. L(+)-Lactate and racemic lactate addition resulted in a reduction of the propionate turnover to 26.9 and 11.6 mmol propionate . l(-1). d(-1), respectively. The propionate turnover was not affected by the addition of sulfate, neither stimulating nor inhibiting, but in the presence of sulfate the electron flow showed a partition into sulfate reduction and methanogenesis. 22% of electrons released by propionate oxidation served to reduce sulfate, while 78% of propionate was still syntrophically degraded with concomitant methane-production. Injection of 1 mM sodium molybdate into the reactor caused an immediate and total inhibition. The biofilm population was stable and could survive a 5.5 months starvation period without significant loss of activity.