Chitin, a beta-1,4-linked polysaccharide of N-acetylglucosamine, is a major structural component of fungal cell walls. Fungi have multiple classes of chitin synthases that catalyse N-acetylglucosamine polymerization. Here, we demonstrate the requirement for a class V chitin synthase during host infection by the vascular wilt pathogen Fusarium oxysporum. The chsV gene was identified in an insertional mutagenesis screen for pathogenicity mutants. ChsV has a putative myosin motor and a chitin synthase domain characteristic of class V chitin synthases. The chsV insertional mutant and a gene replacement mutant of F. oxysporum display morphological abnormalities such as hyphal swellings that are indicative of alterations in cell wall structure and can be partially restored by osmotic stabilizer. The mutants are unable to infect and colonize tomato plants or to grow invasively on tomato fruit tissue. They are also hypersensitive to plant antimicrobial defence compounds such as the tomato phytoanticipin alpha-tomatine or H2O2. Reintroduction of a functional chsV copy into the mutant restored the growth phenotype of the wild-type strain. These data suggest that F. oxysporum requires a specific class V chitin synthase for pathogenesis, most probably to protect itself against plant defence mechanisms.