The following 37 counties are participating in the Pennsylvania Black Fly Suppression Program:
- Adams
- Armstrong
- Berks
- Bradford
- Bucks
- Cambria
- Chester
- Clarion
- Clearfield
- Clinton
- Columbia
- Cumberland
- Dauphin
- Elk
- Fayette
- Forest
- Huntingdon
- Indiana
- Juniata
- Lackawanna
- Lawrence
- Lebanon
- Lehigh
- Luzerne
- Lycoming
- Mifflin
- Montgomery
- Northampton
- Perry
- Schuylkill
- Snyder
- Union
- Venango
- Warren
- Westmoreland
- Wyoming
- York