The respiration of stage-I,-III, and-IV sporangiophores of Phycomyces was measured. The [Formula: see text] of stages-I and-IV is low compared to that of actively growing mycelium. The [Formula: see text] of stage-III sporangiophores is somewhat higher than that of stages-I and-IV. As stage-I sporangiophores elongate their respiration rate increases proportionately. The respiration rate of stage-IV sporangiophores does not change as they elongate. In stage-IV sporangiophores the growing zone and the columella together consume as much oxygen as the rest of the sporangiophore does. Changes in the sporangiophore's growth rate have no effect on its oxygen consumption.