For some applications, such as genome sequencing and high-throughput genotyping with multiple markers, it is necessary to use high-quality genomic DNA. This article describes how to obtain several micrograms of high-quality, cesium chloride-purified DNA from 1 g of Ectocarpus filaments. We also recommend using DNA of this quality for quantitative RT-PCR control reactions. However, simpler, more rapid, kit-based methods are preferable for experiments that involve the treatment of large numbers of individuals, such as genotyping large populations with a small number of markers or PCR screening of large populations.