Nitrification became the dominant nitrogen transformation in a number of lakes which accumulated ammonium, in hypolimnetic water, under aerobic conditions. The timing and duration of this activity varied between lakes but was characterized by decreasing ammonium, and increasing nitrate, concentrations. In Grasmere lake this phase was found to be due to the activity of planktonic chemolithotrophic nitrifying bacteria. The observed nitrate concentration increased during this phase and accounted for up to 15% of the total oxygen deficit of the hypolimnion. At similar in situ temperatures nitrification rates and nitrifying bacterial populations were greater in oxidised sediments than in the water column. Littoral sediments were more important than profundal sediments as sites of nitrification due to higher temperatures and the persistance of oxidising conditions to greater depths into these deposits. Within the littoral zone the physical characteristics of the sediment were important with organic rich deposits sustaining greater rates of nitrification than sandy sediments. Nitrate reduction was the primary determining factor of nitrate concentrations in sediment interstitial waters. Some observations on the culture of chemolithotrophic nitrifying bacteria from lakewater were discussed. Reviews of the methodology for nitrification research and nitrification in the lacustrine environment are presented.