The nature of riboflavin precursors was studied in the yeast Pichia guilliermondii. By means of mutants with blocked GMP-synthetase the purine precursors of riboflavin were shown to belong to guanylic compounds. Accumulation of 2,4,5-triamino-6-oxypyrimidine, 2,5-diamino-6-oxy-4-ribitylaminopyrimidine, 2,6-dioxy-5-amino-4-ribitylaminopyrimidine (DOARAP) and 6,7-dimethyl-8-ribityllumasine occurs in the riboflavin-deficient mutants divided into five biochemical groups. This fact evidences for identity of riboflavin precursors in the yeast P. guilliermondii and Saccharomyces cerevisiae. Synthesis of DOARAP by the washed off cells of the mutants with the blocked lumasine synthetase is strongly inhibited by riboflavin; cycloheximide in the absence of riboflavin has no effect on this process. Consequently, flavinogenesis in P. guilliermondii is regulated according to the type of negative feedback by means of retroinhibition mechanism. A change in the content of flavins in the cells has no effect on synthesis of riboflavin synthetase; at the same time iron deficiency in the cells evokes derepression of this enzyme. Incubation of the cells rich in iron with o-phenantroline or alpha, alpha'-dipyridyl also causes derepression of riboflavin synthetase which is inhibited by cycloheximide. A deficiency of hem in the mutants which need epsilon-aminolevulinic acid does not affect the riboflavinsynthetase activity of the cells. Evidently, in P. guilliermondii a certain form of nonheminic iron might take part in regulating synthesis of riboflavin synthetase and other enzymes participating in riboflavin biosynthesis. Riboflavin overproduction is established to require formation of purines de novo. With the absence of flavinogenesis enzymes derepression a genetic disturbance in regulation of purinic nucleotides biosynthesis results in stimulation of flavinogenesis. The properties were studied for 680 time purified riboflavinkinase from cells of P. guilliermondii as well as for three phosphatases possessing the optimum of the activity at pH 3.5, 5.5 and 8.6, which ARE ABLE OF HYDROLYSING FMN. A change in the content of flavins and iron in the cells has no effect on the activity of riboflavinkinase in this species. Evidently, the mechanisms of riboflavin and flavin nucleotides biosynthesis regulation would be different in P. guilliermondii.