Ion channel genes have been discovered in many microbial organisms. We have investigated a transient receptor potential ion channel gene in fission yeast which has most similarity (46%) in the six predicted transmembrane domains to a polycystic kidney disease related ion channel gene in Drosophila melanogaster. We have shown that this gene (pkd2) is essential for cellular viability and involved in cell growth and cell shape determination. Cells in liquid culture 24 h after pkd2 depletion appeared elongated, average cell length was increased by 31% (compared with the control cells) with 11% of cells containing more than three septae compared with less than 1% for control cells. Additionally, 22% of the cells appeared bulbous and possessed an uneven cell periphery, whilst 5% of the cells appeared to be dead, which was confirmed by trypan blue staining. Over-expression of pkd2 resulted in cell death on liquid and solid media. Microscopic examination of these cells at 16 h following promoter induction suggested cell lysis, with many bent and kinked cells. Significant changes in cellular morphology were observed: about 12% of cells had an altered growth polarization, appearing as small buds growing parallel or at 45 deg to the long axis of cell growth. Average cell length was increased by 8% but multiple septae were not observed. This fission yeast gene appears to be a key signalling component in the regulation of cell shape and cell wall synthesis through an interaction with Rho1-GTPase which was determined by immunoprecipitation. This gene is a candidate for the mechanosensitive ion channel which has been recorded from the plasma membrane of fission yeast¹. A model for the mode of action of this protein in a Ca²⁺ signalling pathway is hypothesized.