1000BASE-LX is a fiber optic gigabit Ethernet standard specified in IEEE 802.3 Clause 38 which uses a long wavelength laser (1270 to 1355 nm), and a maximum RMS spectral width of 4 nm.

1000BASE-LX is specified to work over a distance of up to 5 km over 10 µm single-mode fiber.

1000BASE-LX can also run over all common types of multi-mode fiber with a maximum segment length of 550 m. For link distances greater than 300 m, the use of a special launch conditioning patch cord may be required. This launches the laser at a precise offset from the center of the fiber which causes it to spread across the diameter of the fiber core, reducing the effect known as differential mode delay which occurs when the laser couples onto only a small number of available modes in multi-mode fiber.