The purpose of this paper is
to classify surfaces in Euclidean 3-space with constant Gaussian curvature which
admit non-trivial one-parameter families of isometric immersions preserving the mean
curvature function. It is shown that the Gaussian curvature must be zero and, if the
mean curvature is not constant, then such isometric immersions are some
deformations of the cylinder over a logarithmic spiral.