Flow properties of a bubble plume in density-stratified conditions are studied using planar laser-induced fluorescence (PLIF) flow visualization. Entrained ambient fluid is identified by applying an image intensity threshold to the PLIF images of passive dye tracer injected at the bubble diffuser. The density stratification gradually arrests the entrained ambient fluid, causing detrainment, or peeling, of the continuous phase fluid from the bubble plume core and intrusion of the detrained fluid at a level of neutral buoyancy; bubbles continue to rise above each detrainment zone. The peel and intrusion heights for the first detrainment event above the diffuser are measured from the thresholded PLIF images. The nondimensional frequency of fluctuations in the detrainment and intrusion heights fz/us is measured as 0.35 (where f=frequency; z=height above the diffuser; and us=terminal rise velocity of the bubbles in a quiescent fluid), and this value compares well to the plume wandering frequency for similar experiments in unstratified reservoirs.