Chlamydial cell types are adapted for either extracellular survival or intracellular growth. In the transcriptionally inert elementary bodies, the chromosome is densely compacted; in metabolically active reticulate bodies, the chromatin is loosely organized. Condensation of the chlamydial nucleoid occurs concomitant with expression of proteins homologous to eukaryotic histone H1. When the Chlamydia trachomatis 18-kilodalton histone homolog Hc1 is expressed in Escherichia coli, a condensed nucleoid structure similar to that of chlamydiae is observed with both light and electron microscopy. These results support a role for Hc1 in condensation of the chlamydial nucleoid.