St Anthony being at one time at Rimini, where there were a great number of heretics, and wishing to lead them by the light of faith into the way of truth, preached to them for several days, and reasoned with them on the faith of Christ and on the Holy Scriptures. They not only resisted his words, but were hardened and obstinate, refusing to listen to him.
At last St Anthony, inspired by God, went down to the sea-shore, where the river runs into the sea, and having placed himself on a bank between the river and the sea, he began to speak to the fishes as if the Lord had sent him to preach to them, and said: "Listen to the word of God, O ye fishes of the sea and of the river, seeing that the faithless heretics refuse to do so."
No sooner had he spoken these words than suddenly so great a multitude of fishes, both small and great, approached the bank on which he stood, that never before had so many been seen in the sea or the river. All kept their heads out of the water, and seemed to be looking attentively on St Anthony's face; all were ranged in perfect order and most peacefully, the smaller ones in front near the bank, after them came those a little bigger, and last of all, were the water was deeper, the largest.
When they had placed themselves in this order, St Anthony began to preach to them most solemnly, saying: "My brothers the fishes, you are bound, as much as is in your power, to return thanks to your Creator, who has given you so noble an element for your dwelling; for you have at your choice both sweet water and salt; you have many places of refuge from the tempest; you have likewise a pure and transparent element for your nourishment. God, your bountiful and kind Creator, when he made you, ordered you to increase and multiply, and gave you his blessing. In the universal deluge, all other creatures perished; you alone did God preserve from all harm. He has given you fins to enable you to go where you will. To you was it granted, according to the commandment of God, to keep the prophet Jonas, and after three days to throw him safe and sound on dry land. You it was who gave the tribute-money to our Saviour Jesus Christ, when, through his poverty, he had not wherewith to pay. By a singular mystery you were the nourishment of the eternal King, Jesus Christ, before and after his resurrection. Because of all these things you are bound to praise and bless the Lord, who has given you blessings so many and so much greater than to other creatures."
At these words the fish began to open their mouths, and bow their heads, endeavouring as much as was in their power to express their reverence and show forth their praise. St Anthony, seeing the reverence of the fish towards their Creator, rejoiced greatly in spirit, and said with a loud voice: "Blessed be the eternal God; for the fishes of the sea honour him more than men without faith, and animals without reason listen to his word with greater attention than sinful heretics."
And whilst St Anthony was preaching, the number of fishes increased, and none of them left the place that he had chosen. And the people of the city hearing of the miracle, made haste to go and witness it. With them also came the heretics of whom we have spoken above, who, seeing so wonderful and manifest a miracle, were touched in their hearts; and threw themselves at the feet of St Anthony to hear his words. The saint then began to expound to them the Catholic faith. He preached so eloquently, that all those heretics were converted, and returned to the true faith of Christ; the faithful also were filled with joy, and greatly comforted, being strengthened in the faith. After this St Anthony sent away the fishes, with the blessing of God; and they all departed, rejoicing as they went, and the people returned to the city.
The Little Flowers of St. Francis, Part I, Chapter XL.
Because of the tale, Anthony is often pictured with fish: a reminder that the gospel must be proclaimed even if there is no one to hear but the fish.