The fish always stinks from the head downwards

The freshness of a dead fish can be judged from the condition of its head. Thus, when the responsible part (as the leaders of a country, etc.) is rotten, the rest will soon follow. Gr. ἰχθὺς ἐκ ṯῇς κɛϕαλῇς ὄζɛίν ἄρχɛṯαί, a fish begins to stink from the head.

If the prouerbe be true,‥that a fishe beginneth first to smell at the head,‥the faultes of our seruantes will be layed vppon vs.
[1581 G. Pettie tr. S. Guazzo's Civil Conversation iii. 51]

s.v. Teste, Fish euer begins to taint at the head; the first thing that's deprau'd in man's his wit.
[1611 R. Cotgrave Dict. French & English]

The guilt of criminality attaches to those responsible. ‘Well,’ said the Aga Khan, ‘fish goes rotten by the head.’
[1915 W. S. Churchill Letter 3 Dec. in M. Gilbert Winston S. Churchill (1972) III. Compan. II. 1309]

‘The fish’, as the saying goes, ‘always stinks from the head downwards.’ Last Sunday we deplored Mr. Michael Foot's liking for the street politics of marches and ‘demos’. Since then, a hundred Labour MPs‥have followed their leader's example.
[1981 Sunday Telegraph 3 May 16]

Cardinal Law has to go. The Vatican has to speak up. This fish, as they say, rots from the head.
[2002 Washington Post 19 Mar. A21]

Related to: rulers and ruled

Bibliography of major proverb collections and works cited from modern editions is available here.