<<I want to open a txt file, read it until I find a line that contains
certain info and then append additional data on to that line. The line may
be in the middle of the file. How can I do this?

Depends on how big the file is. For a small file, use the OpenText method
to import into a worksheet, find the line that needs to be appended or
changed, then SaveAs to a new text file.

For a file that is too big to fit on a worksheet, then you will have to use
either "Microsoft Scripting Runtime" or "Windows Script Host Object Model".
Set a reference to one of them in the Tools|References dialog box. Both of
them have FileSystemObject and TextStream objects available. Open the file,
copy lines to a new file until you come to the line to be changed, change
it, write it out, and continue copying lines. Download the Microsoft
Windows Script Help file (SCRIPT56.CHM) for additional details.
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Regards,
Bill Renaud