5-Fluorocytosine (5-FC) has a rapid inhibitory effect on the synthesis of RNA and DNA in the yeast and hyphal form of Candida albicans. 5-FC has a less marked effect on the RNA and DNA content of sensitive strains of Cryptococcus neoformans and has no effect on the nucleic acid content of Aspergillus fumigatus nor of resistant strains of C. albicans and C. neoformans. 5-FC has a slower inhibitory effect on yeast cell number increase and no effect on hyphal length of hyphal phase C. albicans over a 7-h incubation period. Rapidly growing yeasts of C. albicans and C. neoformans decrease in volume and in dry weight per cell. 5-FC prevents this decrease in sensitive strains. These results have been discussed with respect to the known metabolic pathway of 5-FC, the fungistat-ic and fungicidal action of 5-FC and the development of resistance to 5-FC.