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Published online by Cambridge University Press: 20 November 2018
 For an isotropic submanifold  
 ${{M}^{n}}(n\underline{\underline{>}}3)$  of a space form  
 ${{\tilde{M}}^{n+p}}(c)$  of constant sectional curvature  
 $c$ , we show that if the mean curvature vector of  
 ${{M}^{n}}$  is parallel and the sectional curvature  
 $K$  of  
 ${{M}^{n}}$  satisfies some inequality, then the second fundamental form of  
 ${{M}^{n}}$  in  
 ${{\tilde{M}}^{n+p}}$  is parallel and our manifold  
 ${{M}^{n}}$  is a space form.