1) A strain of Escherichia coli mutant 26-43 or Neurospora crassa 18558A, both thiazole moiety of thiamine, responded moderately to 4-methyl-thiazole but not to 4-methyl-requiring 5-methyl-thiazole. 2) Cystine or thiazolidine carboxylic acid maintained the growth of both the strains, while no nutritive activities were found in homocystine, methionine or methionol. 3) By means of paper bioautography, considerable amounts of thiamine and thiazole were detected in the cultures of Neurospora 18558A which had been incubated with cystine or thiazolidine carboxylic acid. 4) In E. coli and Neurospora crassa, evidence was presented that thiazolidine carboxylic acid, cystine and 4-methyl-thiazole make feasible the synthesis of the thiazole moiety of thiamine. This leads a new hypothesis that the synthesis of thiazole takes place through the production of thiazole ring, with its subsequent formation of β-hydroxyethyl group at position 5.