Writing to an existing ASCII file destroys it?!?!

Hi,

I want to modify a single line of an ASCII file which I am reading,
without touching the rest, but this

do i=1, ln
    read(lu,*)
end do
write(lu,'2(F15.6)') a, b

results in a and b being written to line ln (which is correct), and in
all the rest of the file being deleted! Isn't there a way to change
just one line? thanks

Regards,

deltaquattro