Methane is a greenhouse gas with a global warming potential twenty-five times that of carbon dioxide. Animal production is recognized as a significant source of methane to the atmosphere. Dairies on the southern High Plains of New Mexico and Texas are typically open lot, and major sources of methane are enteric emissions from cattle and wastewater lagoons. Uncovered anaerobic lagoons are identified by the U.S. Environmental Protection Agency as a major source of methane in dairy manure management systems. Our objective was to quantify methane emissions from the wastewater lagoons of a commercial dairy located in eastern New Mexico. Research was conducted during six days in August, 2009 at a 3500-cow open lot dairy with flush alleys. Methane concentration over three interconnected lagoons (total area 1.8 ha) was measured using open path laser spectroscopy. Background methane concentration was measured using a back-flush gas chromatography system with flame ionization located upwind in the direction of prevailing winds. Wind and turbulence data were measured using a three-axis sonic anemometer. Emissions were estimated using an inverse dispersion model. Methane concentration over the lagoons ranged from 3 to 12 ppm, and averaged 5.6 ppm; background methane concentration averaged 1.83 ppm. Methane flux density ranged from 165 to 1184 μg m -2 s -1 . Mean daily methane flux density was 402 kg ha -1 d -1 . Per capita methane emission rate averaged 0.211 kg head -1 d -1 . Uncovered anaerobic lagoons were a significant source of methane emitted from this southern High Plains dairy, and lagoons could be a significant control point for emission reduction.