The single calmodulin (CaM) gene and the corresponding cDNA of the chytridiomycete Blastocladiella emersonii were isolated and characterized. The CaM gene is interrupted by three introns and transcribed in a single 0.7-kb mRNA species encoding a predicted protein 91% identical to human CaM. B. emersonii CaM has been expressed in Escherichia coli as a fusion protein with gluthatione S-transferase (GST) and purified by affinity chromatography and cleavage from the GST portion using a site-specific protease. In the presence of Ca²⁺, B. emersonii CaM exhibited a shift in apparent molecular mass similar to that observed with bovine CaM and was able to activate the autophosphorylation of CaM-dependent protein kinase II (CaMKII) from rat brain. CaM expression is developmentally regulated in B. emersonii, with CaM mRNA and protein concentrations increasing during sporulation to maximum levels observed just prior to the release of the zoospores into the medium. Both CaM protein and mRNA levels decrease drastically at the zoospore stage, increasing again during germination. The CaM antagonists compound 48/80, calmidazolium, and W7 were shown to completely inhibit B. emersonii sporulation when added to the cultures at least 120, 150, and 180 min after induction, respectively. All these drugs also inhibited growth and zoospore production in this fungus. The Ca²⁺ channel blocker TMB-8 and the CaMKII inhibitor KN93 completely inhibited sporulation if added up to 60 min after induction of this stage, but only KN93 affected fungal growth. The data presented suggest that the Ca²⁺-CaM complex and CaMKII play an important role during growth and sporulation in B. emersonii.