How to get complementary lines from two text files?

File file1.txt has

123 foo
234 bar
...

File file2.txt has

123 foo
333 foobar
234 bar
...

I want to get all lines in file1.txt and not in file2.txt. The two files are hundreds of MB large and contain non-ASCII characters. What's a fast way to do this?

Solution

Lines, specifically?

fgrep -vxf file2.txt file1.txt

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