The Bureau of Mines evaluated the potential production of phosphate rock from the demonstrated resources of 130 mines and deposits. The evaluation included an estimation of resources, engineering methods, and capital and operating costs, and an economic analysis to determine each operation's average total cost of production over the life of the mine, including a 15-pct discounted-cash flow rate of return on all investments. Quantified but not evaluated in this report are substantial phosphate resources at the inferred and hypothetical resource levels. The 130 mines and deposits contain 6.4 billion tons of recoverable phosphate rock product, about 20 pct from producing mines. At total production costs of under $30 per ton in January 1981 dollars, about 1.3 billion tons of phosphate rock product is potentially available, over 90 pct from producing mines. This study suggests that production from low-cost, high- grade phosphate mines now in operation will decline during the next decade, and new higher cost, lower grade mines will have to be developed to satisfy demand into the next century. In addition to the demonstrated resources evaluted in this study, 7 billion tons of inferred-level and 24 billion tons of hypothetical-level phosphate rock are potentially recoverable, which, in part, includes material containing high amounts of magnesium. Much of this material could likely become available in the near future.