Histones of the green alga Chlamydomonas reinhardfii were prepared by a new method and fractionated by reversed-phase high-performance liquid chromatography. Acid-urea-Triton gel analysis and tritiated acetate labeling demonstrated high levels of steady-state acetylation for the single histone H3 protein, in contrast to low levels on histones H4 and H2B. Twenty percent of histone H3 is subject to dynamic acetylation with, on average, three acetylated lysine residues per protein molecule. Histone synthesis in light-dark-synchronized cultures was biphasic with pattern differences between two histone H1 variants, between two H2A variants, and between H2B and ubiquitinated H2B. Automated protein sequence analysis of histone H3 demonstrated a site-specific pattern of steady-state acetylation between 7 and 17% at five of the six amino-terminal lysines and of monomethylation between 5 and 81 O/O at five of the eight amino-terminal lysines in a pattern that may limit dynamic acetylation. An alga1 histone H 3 sequence was confirmed by protein sequencing with a single threonine as residue 28 instead of the serinez8-alaninez9 sequence, present in all other known plant and animal H 3 histones.