Studies of how flooding affects P availability in natural floodplains are rare. We examined the effects of artificial flooding on P availability in a Georgia floodplain forest. We hypothesized that P availability would increase with flooding, because of the flooding-induced solubilization of phosphate minerals. Field mesocosms (n = 4 per treatment) were flooded with river water according to one of four treatments over ~6 mo: (i) continuously flooded; (ii) flooded for 3 mo and then drained; (iii) flooded for 2 mo, drained for 1 mo, and repeated; and (iv) nonflooded control. Two additional sets of 3-mo flooded-drained mesocosms (n = 4 per set) received added P or N (1 and 10 mg L-1, respectively) with flooding. Soils were collected monthly from both inside and outside of the mesocosms and analyzed by Hedley fractionation; anion-exchange resins (AER) were used to estimate P availability in situ. As indexed by daily supply to AER, P availability was significantly greater in flooded versus control soils, and decreased significantly following drainage, in all treatments at some time during the study. Total P supply to AER was significantly greater in flooded versus control mesocosms regardless of treatment. No significant changes were observed in Fe/Al phosphate fractions. Microbial P was significantly lower in flooded versus control mesocosms during the first 3 mo of flooding and decreased significantly over time in two treatments. In this natural floodplain, biological processes are a more probable explanation for flooding-induced increases in P availability than solubilization of mineral phosphates.