This paper demonstrates that the transmission distance of an ON-OFF-keyed high-speed pseudolinear optical communication system can be dramatically improved by means of runlength-limited coding. The key idea is to impose constraints on transmitted sequences such that the minimum interpulse spacing increases, and the average transmission power decreases. Both effects help suppress intrachannel four-wave mixing, a major nonlinear penalty in pseudolinear long-haul links. Several coding schemes with different properties are designed for various constraints and compared against a prototypical reference system. Finally, numerical simulations are performed for a benchmark system employing differential phase-shift keying. The results obtained indicate that the runlength-limited coding approach can be considered a legitimate alternative to this more advanced modulation format.