Quick compare string to list

[Scooter]

I'm reading in a text file, and for each line in the file, I'm looking
for the existence of phrases from a list. The list contains approx.
120 items currently but will most likely grow. This procedure itself
is not the main function of my program and only grew out of the need
to reformat certain phrases I'm finding in a file before re-outputting
it. But as I suspected, this searching of the lists slows the whole
process way way down. Was looking for ideas of a better way to do
this.

I basically have

mylist=[]
...
code that reads in the flat file into string 'flatfileString'
...
for listitem in mylist:
    if flatfileString.count(listitem):
        ...whatever...I found it.