We have shown that Ca(II) ions, ethylene glycol-bis(beta-aminoethyl ether)-N,N-tetraacetic acid, LaCl3, and six known calmodulin inhibitors shift the yeast-mycelium dimorphic potential of Ceratocystis ulmi. Our data are consistent with the conclusions that Ca(II)-calmodulin interaction is necessary for mycelial growth in C. ulmi and that the absence of this interaction leads to the yeast phase.