We demonstrate the error-free transmission of directly modulated 2.5, 10, and 40-Gb/s coarse wavelength-division-multiplexing/dense wavelength-division-multiplexing signals over negative dispersion fiber (dispersion: -2.5 ps/km/nm @ 1550 nm) without dispersion compensation. The results indicate that the metro core/access network could be implemented cost-effectively by using the proposed negative dispersion fiber and directly modulated lasers. (C) 2004 Elsevier B.V. All rights reserved.