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The precipitation reaction to form struvite (MgNH4PO4 6 H2O) or calcium phosphate from nutrient rich manure supernatants and slurries is limited by the concentration of phosphorus in dissolved form (P-PO4) . Liquid hog manure holds ~10% of total phosphate (TP) as P-PO4, the rest bound in either organic or inorganic solids. In order to increase P recovery from this resource, P must be converted to the dissolved phase where it can be precipitated. The purpose of this experiment was to investigate the effect of anaerobic fermentation and resulting acidification on the abundance of P-PO4 and other struvite constituents in hog manure. Manure (2.5% solids) was stirred anaerobically at 4, 8, and 12 day hydraulic retention times (HRT) in two continuously fed reactors. Anaerobic fermentation was effective in decreasing the pH of manure (from pH 7.4 to 5.8) and caused P-PO4 to increase from 10 to 27% of TP in the 12 d HRT effluent. On a mass balance basis, manure P removable as struvite increased from 16 to 41% assuming all P-PO4 could be successfully removed as struvite.