Abstract The phase diagram of D-BCCD (Deuterated Betaine Calcium Chloride Dihydrate) was determined with elastic neutron scattering under an electric field of 5kV/cm applied along the ferroelectric [bvec] axis. Our results are in good agreement with previous neutron measurements2,3 obtained under a smaller field, and show clearly, as predicted by Perez-Mato1, the increase of the stability range of the commensurate phases which verify δ=even/odd, and more particularly for δ=(2/7), (4/15), (2/9) and (2/11). Moreover, the behaviour of the third order satellite indicates that the static modulation becomes non-sinusoidal below T i −24K. Our study carried out at zero field with a bigger sample6 is confirmed, and contradicts previous structural results on BCCD9.