Sugar beet plants (Beta vulgaris L.) were exposed to long-term incubation in light (34 h) and dark (78 h) and changes in enzyme activity, protein and transcripts levels of cytosolic fructose-1, 6-bisphosphatase (FBPase) in leaves were measured. The enzyme activity in leaves exposed to 34 h light was three times higher than the activity in leaves subjected to 78 h dark condition. Surprisingly, the enzyme activity was still remained stable even in the long-term dark treatment although the activity is lower. The transcript levels of cytosolic FBPase in 78 h dark were expressed equivalent to 10% of the expression occurred in 34 h light. However, the protein levels remained unchanged in long-term incubation with either light or dark. The cytosolic FBPase protein was found to be modified during the long-term dark incubation. The isoelectric pH for the protein spot detected by western blot on the two dimensional PAGE is 5. 4 in light sample while the pH value in dark sample is 6.0. Interestingly, the cytosolic FBPase in long-term dark treated leaves was activated 44% more by incubating in 50mM mannose (sequestering agent of endogenous Pi) comparing to the activity with mannitol treatment as a control. Collectively, these data suggest that the cytosolic FBPase become less active by posttranslational modification during a long-term dark period and this modification probably occurs due to the phosphorylation of the enzyme.