The deoxyribonucleic acid (DNA) content of spermatogenic cells of the decapod crab, Emerita analoga, has been cytophotometrically determined at various stages of development, using Feulgen-stained nuclei. The haploid DNA value is found to be 2.9 x 10–12 g, regardless of the nuclear histone content, which drops to cytochemically indetectable levels by sperm maturity. In determining this value, a precise extinction coefficient for Feulgen-stained nuclei was first determined using chicken, frog and toad erythrocyte nuclei and also nuclei from various rat tissues. The effect of naturally occurring nuclear histone depletion on the quantitative Feulgen reaction has also been examined. Identical hydrolysis curves and Feulgen spectral absorption curves are found for somatic nuclei, which contain a full histone complement, and for mature sperm nuclei, which do not contain histones. Loss of nuclear histone does not lead to a change in total Feulgen dye bound per nucleus, as early, middle and late spermatids have equal DNA contents as reflected by Feulgen binding, and primary spermatocytes contain 4 times this value, as expected from the DNA constancy law. The effect of histones (and other proteins) on quantitative Feulgen microspectrophotometry is discussed.