Hi Eran, The problem is the argument "data" being passed to your routine is 1-dimensional, it needs to be 3-dimensional. For instance you are passing an array to your write_mrc() routine that looks like data = (1,2,3,4,5,6,7,8) instead of data = (((1,2),(3,4)),((5,6), (7,8))) # Dimensions 2 by 2 by 2 There are two solutions. Either you pass in the 3-dimensional array to your routine, or you pass in 1-dimensional array and a second argument which gives the grid size. This second option would look like: def write_mrc(data, zyx_grid_size): print data origin = (1.54, 2.45, 3.45) # XYZ coordinates of array index (0,0,0) step = (.25, .25, .25) # Spacing between grid points import Numeric a = Numeric.array(data, Numeric.Float32) a = a.resize(zyx_grid_size) arrays = [a] import VolumeData g = VolumeData.Array_Grid_Data(arrays, origin, step) path = '/home/eshacham/structures/result.mrc' from VolumeData.mrc import write_mrc2000_grid_data write_mrc2000_grid_data(g, path) Example call: data = (1,2,3,4,5,6,7,8) zyx_grid_size = (2,2,2) write_mrc(data, zyx_grid_size) Notice that the grid size is 3 numbers (zs,ys,xs) the first being the z-axis size. Also the data values in the 1-D array have to be ordered such that x varies fastest, y second fastest, and z slowest. In other words the first xs*ys numbers are the first z plane of data. Tom