The effect of ionic species on the flotation selectivity of apatite was evaluated. Water containing fluoride, calcium, magnesium, and phosphate was used in the flotation experiments carried out in a flotation column for coarse and fine phosphate ore. The selectivity of apatite over ferrous and siliceous gangue was investigated. Coarse and fine ores showed different sensitivities according to the ion concentration. The results clearly showed the harmful effect of the ionic species under study on apatite selectivity. The contaminants responsible for the lowest selectivity ratios for ferrous and silicate gangue were calcium and magnesium, regardless of particle size.