Searching Tables

This subject probably isn't correct, but I'm not sure how else to describe what I want to do.

I have two tables.
Table 1:
Act_Num.....Code
1...........33
2...........23
3...........22
4...........26
5...........20

Table 2:
Part_ID.....Act_Listings
1927..........33 14
5819..........23 26

What I need to do is identify records from Table 2 that do not have an Activity in the Act_Listing field contained in Table 1. So for Part_ID=1927, code 33 exists in Table 1, but code 14 doesn't, so that record should be selected.