A semi-synthetic thiamine-free diet was used on weaned lambs to test the effect of a high sulfur level on the rumen, microbial activity and on the microbial production of thiamine. In vivo and in vitro kinetic studies, as well as the determination of the thiamine concentrations and thiaminase activity in the rumen, were performed during the 16 week experiment. A high sulfur level (0.6%) in the diet, in comparison with a normal sulfur level (0.2%), did not modify the microbial activity of the rumen with the exception of a slightly retarded decrease in the volatile fatty acid (VFA) rumen concentration. The rumen thiamine level and the thiaminase activity were not modified by the dietary sulfur level. In contrast, the rate of sulfate reduction into sulfide in the rumen increased progressively with the 0.6% sulfur diet. In conclusion, a high sulfur level (0.6%) in the diet of sheep did not modify the thiamine status of the rumen. It strongly increased the production of sulfides but an adaptation period of several weeks was required by the rumen microflora to reduce sulfate at a maximal rate. rumen / sulfur / thiamine / synthetic diet Résumé ― Effet d'un régime excédentaire en soufre sur l'activité microbienne du rumen et sur le métabolisme ruminal de la thiamine chez le mouton recevant un régime semi-synthétique dépourvu de thiamine. Un régime semi-synthétique dépourvu de thiamine a été utilisé chez l'agneau * Correspondence and reprints minasique. En conclusion un régime à 0, 6 % de soufre chez le mouton n'a pas d'effet sur la concentration ruminale de thiamine d'origine microbienne. Il provoque une très forte augmentation de la production de sulfures mais celle-ci n'est maximale qu'après une période d'adaptation de la micropopulation du rumen.