Phosphorus has a characteristic electron energy loss with an edge at 160 eV. The Zeiss 902 electron microscope permits images to be recorded at different energy losses which can be digitized and processed to yield a net phosphorus image. For nuclear chromatin, phosphorus is primarily located in DNA, allowing its distribution in chromatin fibers to be mapped. Electron energy loss (EELS) imaging is also known as electron spectroscopic imaging (ESI) and electron energy filtering TEM (EFTEM). Starfish sperm chromatin, which has clearly defined chromatin fibers was fixed, embedded in Lowicryl at low temperature, thin sections prepared and imaged unstained. The 4 grouped images were recorded with a Zeiss 902 energy filtered TEM at 80KV using 5eV windows centered on energy loss values 160, 170 (this image), 120, and 110eV.