Yes, immediately. Letting your bank know there is a missing check out there could save you from a lot of hardship. Check fraud is at an all time high, now more than ever, and there are many different ways to go about it. Altered checks, washed checks, blank checks. They've done it all. Tellers go through extensive fraud training to detect checks that look like they could be fraudulent, but I get around one to four fraud and suspected fraud emails every day from other branches. We try our best, but giving us a heads up could give you the best fighting chance of keeping your account safe.