Mass transfer of oxygen to the microbial cell is very important in hydrocarbon fermentations. One method of increasing mass transfer of oxygen is to increase its partial pressure. Increasing the total pressure of the air-feed will increase the partial pressure of oxygen, but little work has been done in this area. The present study concerns the effects of increased air pressure up to 100 pounds per square inch gauge (p.s.i.g.) on the growth ofCandida lipolyticaonn-hexadecane in an agitated 7-liter pressure reactor. Inhibited growth was observed with increasing pressure. Maximum cell mass yield at 100 p.s.i.g. was about one-third that at 15 p.s.i.g. Inhibition was attributed to a gaseous component fed in with the air stream.