Measurements of H-1-NMR and Se-77-NMR have been conducted with CsHSeO4, in order to clarify dynamics of proton and SeO4 tetrahedron which is related to transport of proton below and above T-c = 401 K. An NMR absorption line is observed in H-1-NMR measurement and its line width decreases with increasing temperature below T-c, due to the motional narrowing. These results indicate that the mobile proton exists even below T-c. A correlation time tau is estimated from the line width for the mobile proton to be about 10(-4) 10(-6) s below T-c. Furthermore it is deduced from Se-77-NMR measurements that the reorientational motion of SeO4 tetrahedron is related to the transport of proton below T-c. (c) 2005 Elsevier B.V All rights reserved.