The general transcriptional repressor Tup1p is known to influence cell development in many fungi. To determine whether the Tup1p ortholog (WdTup1p) of Wangiella dermatitidis also influences cellular development in this melanized, polymorphic human pathogen, the gene (WdTUP1) that encodes this transcription factor was isolated, sequenced and disrupted. Phylogenetic analysis showed that the WdTup1p sequence was closely related to homologues in other polymorphic, conidiogenous fungi. Disruption of WdTUP1 produced mutants (wdtup1Delta) with pronounced growth and cellular abnormalities, including slow growth on various agar media and exclusively as a filamentous morphotype in liquid media. We concluded that WdTup1p represents an important switch regulator that controls the yeast-to-filamentous growth transition. However, detailed observations of the filamentous growth of the disruption mutant showed that the hyphae produced by the wdtup1Delta mutants, unlike those of the wild-type, were arrested at a stage prior to the formation of true hyphae and subsequent conidia production.