I think you will see the way your plot is interpreting your data if you type at your IDL command prompt: IDL> PRINT, FIX(32769) ; convert 32769L to a signed short integer You will see that many, if not all, of your data values > 32768 are interpreted as negative numbers. (And, in all cases, they would be values mush smaller than the value you would expect.) I do not know how you have set up your IDL program to acquire and store this NON-short-integer data. However, if the data values are bigger than what would fit in a 16-bit integer "short" (the IDL "default" for integer data), then you should take steps to store it in a larger datatype. Your INT's should be LONGs, your INTARR's then should preferably be initialized as LONARR's, and any conversion function you use should be LONG( ) instead of FIX ( ). James Jones