A shepherdess asks the seer whether she will be happy in love (Air: "Je ne sais quel ennui me presse") and he replies in the affirmative. Tirésie begins to interpret the song of the birds: he tells his listeners to beware of the wrath of the god of the sea and that Naïs will find true love. Astérion and Télénus decide that the oracle means they must sacrifice the life of their rival to appease Neptune (Chorus: "Aux armes, vengeons-nous").
A young shepherdess seeks guidance from a prophet regarding her romantic fate (Air: "I am unaware of what kind of sadness is affecting me") and he declares that she will be joyful. Tirésie starts to decipher the birdsong: he advises his listeners to beware of Neptune's anger and foresees that Naïs will discover true love. Astérion and Télénus decide that the oracle means they must sacrifice their adversary's life to appease Neptune (Chorus: "To arms, avenge ourselves").