Abstract 1. 80-S ribosomes and polyribosomes can be obtained in good yields from the flagellate protozoan Crithidia oncopelti. 2. Investigations revealed that in contrast to the cytoplasmic ribosomes from other eukaryotic cells, ribosomes from C. oncopelti readily dissociated into subunits following a reduction in the Mg 2+ concentration. At 1 mM Mg 2+ all ribosomes not carrying nascent peptides dissociated into 60-S and 40-S subunits. Polyribosomes dissociated at 0.1 mM Mg 2+ giving 45-S and 33-S subunits. 3. The large ribosomal subunit unfolded from a 60-S form in 1 mM Mg 2+, through a 50-S form to the 45-S form which is stable in 0.1 mM Mg 2+. Over the same range of Mg 2+ concentration the small subunit was converted from a 40-S form to a stable 33-S form.