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 the cup-shaped nucleus surrounding the acrosome. Flagellar axonemes are also present. Chromatin fibers in the nucleus are clearly visualized.