Hi Matt, Like Bruce said, ifeffit always reads in the whole data file into data arrays. We could change that.... If you're running out of data space, you can remove temporary arrays: macro readbigfile file group read_data($1, group=_tmp_, label='c1 c2 c3 c4') set $2.energy = _tmp_.c1 set $2.i0 = _tmp_.c2 set $2.fe_01 = _tmp_.c3 # etc erase @group _tmp_ end macro readbigfile 'mydatafile.dat' 'mygroup' (which will read everything and save what you want). This will potentially use up a lot of space, which should then be freed. I think that athena does this sort of trick (read to temporary variables, then erase most of it) when reading in data for multi-element detectors. (If you work hard at this and have large enough data sets, you'll eventually bring ifeffit's crude data management to its knees). For what it's worth, the 'narrays' argument is for weirdly formatted datafiles where 1 line / row does not correspond to 1 data point. Oddly enough, it happens that data sometimes gets written to ASCII files like this: energy1 i01 it2 energy2 i02 it2 energy3 i03 it3 energy4 i04 it4 energy5 i05 it5 energy5 i05 it5 Apparently, that's from back in the days when newline characters were considered wasteful. --Matt