The stage of cellular respiration that is anaerobic is known as glycolysis. This process occurs in the cytoplasm of the cell and does not require oxygen to take place.
During glycolysis, one molecule of glucose is broken down into two molecules of pyruvate, producing a small amount of ATP in the process. As the initial step of cellular respiration, glycolysis can occur regardless of whether oxygen is present or not, making it an anaerobic process. In the absence of oxygen, the pyruvate can then be further processed via fermentation, allowing the cell to continue generating ATP.