Echinoderm sperm nuclei are unusual in that chromatin fibers are clearly seen, allowing their diameter to be accurately determined. Here, sperm from the starfish P. miniata were treated to remove the outer membrane, cryo-fixed by plunging into liquid ethane, freeze-substituted in acetone and low-temperature embedded in Lowicryl K11M. Thin sections were stained with uranyl acetate, and examined using a FEI CM10 TEM operated at 100KV. This image shows a somewhat swollen nucleus that has separated from other cell components containing clearly defined chromatin fibers.