A light transmittance technique using a dual optical fiber probe was used to measure the local solids holdup in a three-dimensional gas-liquid-solid fluidized bed. The bubble can be clearly identified in the same signal, thus permitting the simultaneous determination of the local solids holdup profile in the wake of a single bubble and the rise velocity and chord length of the bubble. The solid holdup behavior was studied in the wake of single bubbles rising in a liquid-solid fluidized bed for different liquid velocities, particle sizes and bubble sizes. Over the range investigated in this study, the wake solids holdup was found to decrease with decreasing mean solids holdup in the bed, to depend only slightly on bubble Reynolds number and to decrease with increasing particle size.