We propose a new nonlinear shell model of Koiter's type, i.e. one that combines membrane and flexural strains, which can be used in the case where the middle surface of the undeformed shell is "almost spherical", in the sense that its Gaussian curvature K-epsilon and mean curvature H-epsilon satisfy K-epsilon = H-epsilon(2) + O (epsilon(2)), where 2 epsilon denotes the thickness of the shell. (C) 2016 Academie des sciences.