We investigate the use of a run-length-limited (RLL) sliding-block line code to reduce the effects of intersymbol interference in high-speed (multi-Gb/s) data transmission over the direct-detection single-mode optical fiber channel. In addition to their well-known use for synchronization purposes, line codes can be designed to eliminate certain worst-case patterns that would otherwise preclude reliable data transmission at high rates and very long fiber lengths. The code considered here prohibits isolated "one's" (the 010 pattern) from appearing in the transmitted sequence. Simulation results indicate that this code, coupled with a simple compensation scheme at the receiver, can significantly increase dispersion-limited data rates and/or fiber lengths for both externally modulated and directly modulated systems. While our emphasis is on one particular line code, we discuss sliding block coding for the optical fiber channel in a general context.>