Enteroblastic phialidic conidiation by the corn pathogen Fusarium verticillioides (teleomorph Gibberella moniliformis) produces abundant, mostly single-celled microconidia in distinctive long chains. Because conidia might be critical for establishing in planta associations, we characterized a spontaneous F. verticillioides conidiation mutant in which phialides were incapable of enteroblastic conidiogenesis. Instead of producing a conidium, the phialide apex developed a determinate, slightly undulating, germ tube-like outgrowth, in which nuclei rarely were seen. Electron microscopy showed that the apical outgrowth possessed a thick, rough, highly fibrillar outer wall layer that was continuous with the thinner and smoother outer wall layer of the phialide. Both the inner wall layer and plasma membrane also were continuous between the apical outgrowth and phialide. The apical neck region of mutant phialides lacked both a thickened inner wall layer and a wall-building zone, which were critical for conidium initial formation. No indication of septum formation or separation of the apical outgrowth from mutant phialides was observed. These aberrations suggested the apical outgrowth was not a functional conidium of altered morphology. The mutation did not prevent perithecium development and ascosporogenesis. Genetic analyses indicated that a single locus, designated FPH1 (frustrated phialide), was responsible for the mutation. The conidiogenesis mutants were recovered only during certain sexual crosses involving wild-type conidiating parents, and then only in some perithecia, suggesting that mutation of FPH1 might be meiotically induced, perhaps due to mispairing between homologous chromosomes and deletion of the gene from a chromosome.