In this research, the effect of bubble size distribution on the performance of column flotation of fine particles of apatite was investigated. The Brazilian apatite ore sample used in this study was low grade (18.38% P2O5 content) with a Sauter mean diameter of 13.95 mu m. Bubbles were measured in a system consisting of a high-speed digital camera and software for image analysis. In addition, probes were used for shooting bubbles. The effect of the presence of mineral particles on the bubble size distribution was also investigated comparing the bubble size in air-water and air-water-ore systems. The presence of ore particles adhered to the bubble surface, prevented bubble coalescence, consequently reducing the bubble size, compared to the air-water system. The flotation tests were performed for different bubble size distributions: small bubbles (d(32) < 300 mu m), medium-sized bubbles (300 < d(32) < 1000 mu m) and large bubbles (d(32) > 1000 mu m). The best bubble size range for flotation of fines particles of apatite was identified and the images from probes helped us to correlate the performance results (P2O5 content and recovery) with the flotation mechanisms. The highest P2O5 content was obtained when the flotation of fine apatite was performed with small bubbles (d(32) < 300 mu m), however, the P2O5 recovery was very low (40%) at this condition. The highest P2O5 recovery was obtained in flotation using larger bubbles (d(32) > 1000 mu m) but with low P2O5 content. The best flotation performance, simultaneously considering both responses (P2O5 content and recovery), was obtained with medium-sized bubbles. (C) 2019 Elsevier B.V. All rights reserved.