| 57th Prix de l'Arc de Triomphe | |
| Horse Race: | Prix de l'Arc de Triomphe |
| Location: | Longchamp, Paris, France |
| Distance: | 2400m (7,900feet) |
| Winning Horse: | Alleged (IRE) |
| Winning Time: | 2:36.1 |
| Final Odds: | 14/10 favourite |
| Winning Jockey: | Lester Piggott |
| Winning Trainer: | Vincent O'Brien |
| Winning Owner: | Robert Sangster |
| Previous: | 1977 |
| Next: | 1979 |
The 1978 Prix de l'Arc de Triomphe was a horse race held at Longchamp on Sunday 1 October 1978. It was the 57th running of the Prix de l'Arc de Triomphe.
The winner was Alleged, a four-year-old colt trained in Ireland by Vincent O'Brien and ridden by Lester Piggott. Alleged, the 14/10 favourite, defeated the mare Trillion by two lengths, with Dancing Maid a further two lengths back in third. Alleged became the sixth horse to win the race twice after Ksar, Motrico, Corrida, Tantieme and Ribot. The winning time was 2:36.1.
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| Pos. | Marg. | Horse | Age | Jockey | Trainer (Country) | |
|---|---|---|---|---|---|---|
| 1 | Alleged | 4 | Lester Piggott | Vincent O'Brien (IRE) | ||
| 2 | 2 | Trillion | 4 | Willie Shoemaker | Maurice Zilber (FR) | |
| 3 | 2 | Dancing Maid | 3 | Freddy Head | Alec Head (FR) | |
| 4 | Frere Basile | 3 | Philippe Paquet | Bernard Secly (FR) | ||
| 5 | Guadanini | 4 | Henri Samani | Robert Carver (FR) | ||
| 6 | hd | Montcontour | 4 | J-L Kessas | Maurice Zilber (FR) | |
| 7 | Noir et Or | 3 | Maurice Philipperon | John Cunnington, Jr (FR) | ||
| 8 | Gay Mecene | 3 | Jean-Claude Desaint | Alec Head (FR) | ||
| 9 | Dancing Master | 3 | Gerard Dubroeucq | J P Gallorini (FR) | ||
| 10 | Lido | 5 | Albert Klimscha, Jr | C Seiffert (GER) | ||
| 11 | nk | Julio Mariner | 3 | Edward Hide | Clive Brittain (GB) | |
| 12 | hd | Acamas | 3 | Yves Saint-Martin | François Mathet (FR) | |
| 13 | 1 | Monseigneur | 4 | Tony Murray | François Boutin (FR) | |
| 14 | hd | Turville | 3 | Alfred Gibert | Aage Paus (FR) | |
| 15 | nk | Shafaraz | 5 | M Planard | Patrick Biancone (FR) | |
| 16 | shd | Galiani | 3 | Alain Lequeux | Maurice Zilber (FR) | |
| 17 | 2 | Exdirectory | 3 | Willie Carson | Paddy Prendergast (IRE) | |
| 18 | 6 | Dom Alaric | 4 | Alain Badel | Olivier Douieb (FR) |
Further details of the winner, Alleged.