Indeterminate narrow-leafed lupin (Lupinus angustifolius L. cv. Merrit) was exposed to enriched atmospheric CO2 during pod-filling to enhance the availability of carbon resources for pod-filling in order to determine whether or not seed-filling, yield, and harvest index are limited by the availability of photosynthetic assimilate. Plants were grown in a glasshouse and the flowers painted with an aqueous solution containing either N6-benzylaminopurine (BAP) or no BAP to generate 2 different numbers of pods per plant. From the time when pods began to fill seeds (≥5 mg/seed) until maturity, plants were exposed to either ambient (350-360 μL/L) or enriched (700 μL/L) CO2 by enclosing them in 2 transparent, box-shaped tunnels with similar temperatures, light, and water conditions. Whether or not BAP was applied to flowers, CO2 enrichment increased the final number of pods and the number of pods that filled large seeds (>150 mg) by 20-22 pods/plant. Enriched CO2 reduced to zero the number of pods that had small seeds (≥30-80 mg) and reduced the number of pods with unfilled seeds from 16 to 1 pod/plant. This increased seed yield per plant by 44-66%, but did not affect the harvest index. Harvest index was unchanged because enriched CO2, while increasing pod-filling, also increased pod set and dry matter accumulation on the developing branches. This indicates that an increased availability of carbon resources during-pod filling changed the allocation of assimilates by filling small seeds and producing new branches. The 47-56% increase in dry matter per plant was reflected in the increase in seed yield, which occurred largely through an increased number of pods and seeds per plant. These data support the idea that seed-filling and hence seed yield in well-nodulated, indeterminate narrow-leafed lupin is limited by carbon resources at the stage when the plant is most source-limited, which is during podset and pod-filling.