Radium isotopes have been measured in Mississippi River water at several locations between St. Francisville and Venice, Louisiana. Measured activities average 11.1 dpm/100 L (disintegrations per minute per 100 L) for dissolved 226Ra with a 228Ra/226Ra activity ratio of 1.04 and a 224Ra/228Ra activity ratio of 1.06 for the section of river above a phosphogypsum waste-discharge input. Down river from this site, the dissolved 226Ra activity averaged 22.4 dpm/100 L with a 228Ra/226Ra activity ratio of 0.55 and a 224Ra/228Ra activity ratio of 0.87. Chemical and isotopic reequilibration of the radium in the waste with that in the river water and suspended sediments occurs very rapidly. Fluxes of 226Ra are calculated to be 2.44¿1014 dpm/yr (disintegrations per minute per year) for the main stem of the Mississippi during phosphogypsum disposal compared to 0.98¿1014 dpm/yr when no phosphogypsum disposal is occurring. 228Ra flux is calculated to be 1.06¿1014 dpm/yr during disposal and 1.01¿1014 dpm/yr in the absence of disposal. The radium flux is estimated to be 45% in the dissolved state and 55% in the adsorbed state. ¿ American Geophysical Union 1991