The Public Library of Science presents PloS Biology, a new journal featuring "works of exceptional significance in all areas of biological science, from molecules to ecosystems, including works at the interface with other disciplines." The Public Library of Science -- as a nonprofit organization "committed to making the world's scientific and medical literature a freely available public resource" -- offers the online version of _PloS Biology_ entirely for free. Readers will find research papers and synopses, editorials, book reviews, and other familiar elements of a scientific journal. PloS Biology also provides two online forums: the Journal Club for postdocs and graduate students and Community Page for organizations and societies. The journal also explores unsolved mysteries, each month highlighting a poorly understood but important topic in the biological sciences.