Fern gametophyte is a good model system to investigate signal transduction in plant cells. In this work, we examined whether CDPKs are involved in the mechanisms of spore germination of the fern Osmunda japonica. A protein extract from the spores included four CDPK isoforms with relative molecular weights of 56, 53, 49, and 47 kDa, as detected by immunoblot analysis, and they showed CDPK-like activities, as detected by in-gel protein-kinase assay. It was also found that the inhibitors effective on CDPKs, such as a general protein kinase inhibitor, K252a, and a calmodulin antagonist, W-7, largely suppressed the spore germination, and that many proteins of the spores were phosphorylated in vivo in a calcium dependent manner in the period when the spores require external $Ca^{2+}$ for the germination. Furthermore, we showed that Sr$^{2+}$ and Mn$^{2+}$, which could substitute for $Ca^{2+}$ in the spore germination, were also able to activate the Osmunda CDPKs. From these results, we concluded that CDPKs would participate in the spore germination of O. japonica.aponica.