Frog Mill Ait is an island in the River Thames

River Thames

The River Thames flows through southern England. It is the longest river entirely in England and the second longest in the United Kingdom. While it is best known because its lower reaches flow through central London, the river flows alongside several other towns and cities, including Oxford,...

 in England between the villages of Medmenham

Medmenham

Medmenham is a village and civil parish in the Wycombe district of Buckinghamshire, England. It is on the River Thames, about three and a half miles southwest of Marlow and three miles east of Henley-on-Thames....

, Buckinghamshire

Buckinghamshire

Buckinghamshire is a ceremonial and non-metropolitan home county in South East England. The county town is Aylesbury, the largest town in the ceremonial county is Milton Keynes and largest town in the non-metropolitan county is High Wycombe....

 and Hurley

Hurley, Berkshire

Hurley is a village and civil parish in the English county of Berkshire.Hurley is famous for its proximity to the River Thames. It is also within easy reach of Henley-on-Thames, Oxford and London. The parish includes the villages and hamlets of Cockpole Green, Warren Row, Knowl Hill, Burchett's...

, Berkshire

Berkshire

Berkshire is a historic county in the South of England. It is also often referred to as the Royal County of Berkshire because of the presence of the royal residence of Windsor Castle in the county; this usage, which dates to the 19th century at least, was recognised by the Queen in 1957, and...

. It is situated on the reach above Hurley Lock

Hurley Lock

Hurley Lock is a lock and weir on the River Thames in England, situated in a clump of wooded islands close to the village of Hurley, Berkshire. The lock was first built by the Thames Navigation Commissioners in 1773....

.

The island is named after a mill on the bank here. Frog Mill Ait and the adjacent Black Boy Island

Black Boy Island

Black Boy Island is an island in the River Thames in England between the villages of Medmenham, Buckinghamshire and Hurley, Berkshire. It is situated on the reach above Hurley Lock....

presented a problem to navigation because the towpath was on the other side of the island from the main navigation channel. Hence tow lines had to sweep over the islands, often with the result that barges were pulled onto the shore.

The name "Poisson Deux" is associated with the area, and this is believed to derive from fish ducts or traps in the river here.