An online fluorometer designed for following intracellular NAD(P)H was used to monitor aerobic sludge digestion experiments. The fluorescence showed an initial rise to a high plateau, a sharp decline after staying at the plateau for 20-60 h, and a trailing very slow decrease. The characteristic fluorescence profile was shown to result mainly from the solids-associated fluorescence, after ruling out other factors such as pH, temperature, and supernatant fluorescence. The fluorescence profile was, however, not a mere result of the decreasing solids concentration. The varying sludge viability and population composition (e.g., the decay of heterotrophs and the increasing fraction of nitrifiers) played important roles. The fluorescence profile correlated well with the profile of the viable heterotrophic cell number concentration evaluated with TSB-agar plates. The initial increase of the number concentration was attributed to the growth of multiple small bacteria from the lysate of each large microorganism, which was demonstrated in the experiments with baker＇s yeast as the starting culture for digestion. The fluorescence profiles observed in the yeast experiments were similar to those in the sludge experiments. Responding to glucose additions and the switch from aerobic to anaerobic conditions, the yeast systems showed typical step increases of fluorescence as expected from the change of NAD(PH level associated with heterotrophic metabolism. However, no such fluorescence responses were detectable in the sludge digestion systems. NAD(P)H were thus uncertain to be responsible for the online fluorescence observed. Nonetheless, the initial fluorescence plateau corresponded to the period of rapid digestion and, for the plant studied, the EPA regulation criteria of VSS reduction >38% and/or SOUR <1.5 mg of O-2 (g of TS)(-1) h(-1) were satisfied at the end of the plateau. The online fluorescence provides an effective means of monitoring the aerobic sludge digestion process.