Seasonal variability in emission of nitrous oxide (N2O) from soil was studied by monitoring N2O emissions from six Iowa soils for 12 months (mid-April, 1979 to mid-April, 1980). The soils studied were representative of soils used extensively for corn and soybean production in north-central Iowa. They had been planted to soybeans and had received no N fertilization. The amounts of N2O--N evolved from these soils in 12 months ranged from 0.34 to 1.97 kg ha-1 and averaged 1.20 kg ha-1. A substantial proportion (average, 67%) of the N2O evolved in 12 months was produced during the summer months of the study period, whereas only a small proportion (average, 2%) was produced during the winter months. The corresponding proportions for the spring and fall months were 18% and 13%, respectively.