The gas exchange characteristics of asparagus shoots at various stages of development are described. Reassimilation in spears conserved 50-100% of the carbon dioxide produced in respiration. Refixation was influenced by light intensity and CO2 concentration in the surrounding air, but not by oxygen concentration. The absence of an inhibitory effect of oxygen on refixation is attributed to suppression by high levels of CO2 inside the spear. Assimilation of radioactive CO2 in both the spear and fern forms of asparagus was by the C3 pathway. Fern forms displayed more usual C3 properties, with photosynthesis and CO2 compensation points showing sensitivity to oxygen concentration.