For various reasons many dairy farms still feed animals with excessive P in diets. We report the results of using a fecal P test to assess if P overfeeding is occurring on farms. Feed and fecal samples were obtained from >90 commercial dairies in the Northeast and Mid-Atlantic regions. Inorganic P (Pi) concentrations in 0.1% HCl extracts of feces closely reflect dietary P changes whereas extractable organic P (Po) plus residual P was independent of dietary P changes. Dairy feces consist of P that is in: (i) undigested feed residues, (ii) microbial residues and sloughed tissues, and (iii) as digested but not absorbed. The last is most important concerning the case of P overfeeding and the idea of fecal P testing. The greater the excess P in diets, the higher the proportion of digested but not absorbed P in feces. We assume that much of the digested but not absorbed P in feces is soluble in 0.1% HCl, whereas the remaining (P in undigested feed residues, microbial residues, and sloughed tissues) is organic with limited solubility. Thus, using 0.1% HCl as an extractant enables us to measure the relative magnitude of the digested but not absorbed P. Its potential usefulness as a management tool is apparent. If a benchmark value is designated to represent adequate P status, we would be able to assess whether excess P intake is occurring in a herd by measuring 0.1% HCl extractable Pi in feces and comparing it with the benchmark. The fecal P test proposed here has the following features: Sample collection is easy and non-invasive, and the laboratory procedures are straighforward and inexpensive. From an environmental perspective, 0.1% HCl-extractable Pi is mostly labile and includes much of the P fraction that is most vulnearable to potential losses after the manure is land applied.