The chemical composition of yeast and mycelial cells of three strains ofHistoplasma capsulatum was analyzed and is expressed as per cent dry weight. Cultures were grown in a liquid synthetic medium, mycelial cells at 25°C and yeast at 37°C on gyrotory shakers. After 7 days, deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and protein were higher in the yeast cells while mycelial cells contained more lipid and carbohydrate. The components of one strain were also studied at different stages of growth. The DNA in both yeast and mycelial cells remained relatively constant, but other components varied with the age of culture. In yeast cells the RNA level was 6.8 % at 2 days and then declined sharply remaining constant around 3.5 %. A protein content of 29 % on day 2 decreased gradually to 19 % on day 14. An initial lipid content of 21 % rose to 33 % on day 5 and then decreased. Similarly, an initial carbohydrate level of 17 % rose to 25 % on day 7 and then declined. The mycelial cells contained 4 % RNA up to 10 days followed by a slight decline to 3 % on day 14. A protein content of 20 % on day 5 increased to 24 % on day 10 and then decreased to 15 % on day 28. The lipid content of 33 % on day 5 rose to 38 % on day 7 and then decreased gradually. The carbohydrate level of 20 % at 5 days increased to 38 % on day 10 and declined gradually to 27 % after 28 days.