The KEM1/XRN1 gene was originally identified because of its functions in microtubule-mediated processes, and is also known to be a major cytoplasmic 5'-3' exoribonuclease gene, which is involved in RNA turnover. Here we present evidence that KEM1 plays a role in filamentous growth. In Saccharomyces cerevisiae, the filamentation signalling shares multiple components of the MAP kinase cascade (STE7, STE11, and KSS1) and the transcription factor STE12 with mating process. Both haploid invasive growth and diploid pseudohyphal growth were found to be greatly impaired in kem1 mutant strains. KEM1 affected the level of FLO11 transcripts and the expression of the filamentation-associated reporter genes, Ty1-lacZ and FLO11-lacZ. Suppression analysis implies that KEM1 does not affect the RAS/PKA pathway, but that it possibly functions downstream of the MAP kinase pathway during filamentation.