A soluble protein was isolated from Mougeotia by chloropromazine-sepharose 4 B affinity chromatography. The protein matches the properties of calmodulin in terms of heat stability, Ca(2+)-dependent electrophoretic mobility in sodium-dodecyl-sulfate polyacrylamide gels, and its ability to activate cyclic nucleotide phosphodiesterase in a Ca(2+)-dependent manner. Phytochrome-mediated chloroplast reorientational movement in Mougeotia was inhibited by the calmodulin antagonist trifluoperazine, a hydrophobic compound, or N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), a hydrophilic compound; 50% inhibition (IC50) of chloroplast movement is caused by 20-50 μmol l(-1) trifluoperazine or 100 μmol l(-1) W-7. The Ca(2+)-calmodulin may act as an intermediate in the chloroplast reorientational response in Mougeotia governed by phytochrome.