A potent sugar sensor gene (g9105) was screened from the genomic data of the cellulase-producing fungus Acremonium cellulolyticus. The transcriptional level of g9105 in the SA49 transformant, which carried the knockdown RNA interference (RNAi) construct, was less than 10 % compared with the parental YP-4 strain. The hairpin-type RNAi construct could be useful for this fungal gene knockdown. Changes in cellulase productivity and protein secretion between these two strains were not observed. The numbers of hyphal tips at subapical branching site were counted for the SA49 and YP-4 strains incubated with potato-dextrose medium at 30 degrees C for 4 days. The hyphal branching ratio of the SA49 strain was higher than that of the YP-4 strain. The present results suggest that the potent sugar sensor gene in A. cellulolyticus could be related with hyphal branch formation.