Lake Bonney, a permanently ice-covered Antarctic lake, has a middepth maximum N2O concentration of 41.6 micromoles N (>580,000% saturation with respect to the global average mixing ratio of N2O) in its east lobe, representing the highest level yet reported for a natural aquatic system. Atmospheric N2O over the lake was 45% above the global average, indicating that this lake is an atmospheric source of N2O. Apparent N2O production (ANP) was correlated with apparent oxygen utilization (AOU), and denitrification was not detectable, implying that nitrification is the primary source for this gas. The slope of a regression of ANP on AOU revealed that potential N2O production per unit of potential O2 consumed in the east lobe of Lake Bonney is at least two orders of magnitude greater than reported for the ocean. The maximum yield ratio for N2O [ANP/(NO2(-) + NO3-)] in Lake Bonney is 26% (i.e. 1 atom of N appears in N2O for every 3.9 atoms appearing in oxidized N), which exceeds previous reports for pelagic systems, being similar to values from reduced sediments. Areal N2O flux from the lake to the atmosphere is >200 times the areal flux reported for oceanic systems; most of this gas apparently enters the atmosphere through a small moat that occupies approximately 3% of the surface of the lake and exists for approximately 10 weeks in summer.