Despite the high oil and protein content of the seeds, the agronomic potential of Lupinus mutabilis Sweet, a South American lupin species, is limited because of its low seed yield and its uncertain maturity. Dry matter accumulation and partitioning, and seed yield, of 2 genotypes (early-maturing LM34 and late-maturing LM268) were studied in 2 seasons at 2 densities at Lusignan, France. Total dry matter at harvest was, on average, 680 g/m2. The mainstem and first-order branches mainly contributed to total plant biomass but pods were only produced on the mainstem. On average, across the different crops, the maximum leaf area index (LAI) was 2· 8. The duration of LAI above 2·5 was short and not correlated with the total dry matter production. The partitioning of the dry matter varied according to the genotype. LM34 showed enhanced pod growth, its harvest index (HI) was 0·32, and seed yield averaged 1·28 t/ha. LM268 showed enhanced vegetative growth, both HI and seed yield were lower (0·16 and 1·13 t/ha, respectively), and whole plant maturity was never reached. For both genotypes during the pod filling, no remobilisation of assimilates occurred from stems to pods.