Fox’s “Fighting 300” is a list of Union regiments that suffered at least 130 deaths in battle, and a few that did not quite reach that number, but which suffered losses which by percentage were considered worthy of inclusion. The list appears in William F. Fox, Lt. Col. U. S. V., Regimental Losses in the American Civil War, 1861-1865: A Treatise on the extent and nature of the mortuary losses in the Union regiments, with full and exhaustive statistics compiled from the official records on file in the state military bureaus and at Washington. A disproportionate number of these regiments served in the Army of the Potomac and many were present at Gettysburg, including the cavalry regiment that suffered the greatest battle loss (the First Maine Cavalry) and infantry regiment (the 5th New Hampshire).
As Fox stated:
“It is not claimed that these are the Three Hundred Fighting Regiments of the Army; but, that they are three hundred regiments which evidently did considerable fighting. There were, undoubtedly, others which did equally good or, perhaps, better fighting, and their gallant services will be fully recognized by the writers who are conversant with their history.”
A list of the Fighting 300 Regiments present at Gettysburg as members of the Army of the Potomac can be browsed here: Fighting 300 Regiments. These regiments also are denoted with a special graphic on their respective regimental monument pages.