A 4-year interdisciplinary study was performed to compare two organically managed sheep production systems, using 118 ewes and 24 ha each. The systems differed in the ewes’ reproduction rhythm: one lambing per ewe per year (1L/1Y), with the aim of balancing feed self-sufficiency and lamb marketing periods v. three lambings over 2 years (3L/2Y), with the aim of maximising ewes’ productivity. The sustainability was evaluated through ewes’ reproductive performance, lamb growth rate, carcass characteristics and quality, animal health, forage and feed self-sufficiency, soil mineral balance and gross margin. General animal health was assessed by recording ewe and lamb mortality and putative cause of death. Nematode digestive-tract strongyles parasitism was studied using faecal egg counts and necropsies. Carcass quality was assessed by recording carcass weight, conformation and fatness, and colour and firmness of subcutaneous fat. Thirty-three percent of 3L/2Y ewes lambed twice a year against 4% of 1L/1Y. Mean ewe productivity was 161.3% and 151.0% in 3L/2Y and 1L/1Y, respectively, and it was more variable between years in 3L/2Y. Average concentrate feed consumption and the corresponding cost per ewe were higher in 3L/2Y than in 1L/1Y (156 v. 121 kg, and h49.5 v. h39.3, respectively). Finally, average gross margin was lower in 3L/2Y than in 1L/1Y (h59 v. h65 per ewe, respectively). Even in year 2002, when ewe productivity was highest in 3L/2Y (193%), gross margin was not different between systems (h90 v. h86 per ewe in 3L/2Y and 1L/1Y, respectively), because of higher concentrate costs in 3L/2Y. The 3L/2Y animals presented a lower health status, with a higher lamb mortality ( P,0.05) and a higher digestive-tract strongyles and coccidia parasitism level ( P,0.05). Lamb carcass conformation, fatness and fat colour were not different between systems, but carcass weight and subcutaneous dorsal fat firmness were lower in 3L/2Y lambs than in 1L/1Y lambs ( P,0.05 and ,0.001, respectively). Intensification in an organically managed sheep system, through an increased reproduction rhythm, thus did not lead to better economic results and proved riskier, more variable and more difficult to manage, and so less sustainable. The less intensive system (1L/1Y) was both highly efficient from the animal standpoint and highly feed self-sufficient. The technical and economic results of this system were better than those of organic private farms in the same area and matched those of non-organic farms.