The drag force (F-d) on bio-coated particles taken from two laboratory-scale liquid-solid circulating fluidized bed bioreactors (LSCFBBR) was studied. The terminal velocities (u(t)) and Reynolds numbers (Re-t) of particles observed were higher than reported in the literature. Literature equations for determining ut were found inadequate to predict drag coefficient (C-d) in Re-t > 130. A new equation for determining F-d as an explicit function of terminal settling velocity was generated based on Archimedes numbers (Ar) of the biofilm-coated particle. The proposed equation adequately predicted the terminal settling velocity of other literature data at lower Ret of less than 130, with an accuracy >85%. (C) 2010 American Institute of Chemical Engineers AIChE J, 56: 2598-2606, 2010