Gallosilicate zeolites, termed ECR-10, have been crystallized from gel compositions aCs(2)O:bNa(2)O:Ga2O3:cSiO(2):dH(2)O gels with 0.5 < a < 0.8, 1.0 < b < 2.0, 2 < c < 2.5, and 50 < d < 100. ECR-10 has a typical composition of Na2.7Cs2.1Si6.9Ga4.8O24. nH(2)O (Si:Ga = 1.4), a cubic unit cell with a(o) similar to 14.9 Angstrom and the same framework topology as that of the aluminosilicate zeolite rho (RHO-framework), The Si:Ga ratios typical of ECR-10 are substantially smaller than the Si:Al ratios of zeolite rho materials; attempts to synthesize either ECR-10 or rho in overlapping composition domains have not, to date, been successful. Distance least-squares modeling was used to demonstrate the feasibility of accommodating the required number of framework gallium atoms in the unit cell observed for ECR-10. Subsequent refinement of this trial structure for the dehydrated zeolite was achieved by Rietveld analysis of monochromatic powder neutron diffraction data. The derived structural parameters are compared with those of other RHO-framework materials. Framework gallium substitution leads to a substantial distortion of the eight-ring windows, reducing the effective pore dimensions and suggesting the possibility of pore dimension control via partial framework cation substitution.