If intellectual men sometimes ignored his preaching, God intervened to show that was worthy of respect, giving signs through dumb animals. In the area near Padua, there was once a group of heretics who criticised and ridiculed his preaching; the Saint went to the edge of a river, looked in the distance, and said to the heretics so that everyone would hear:
"From the moment in which you proved yourselves to be unworthy of the Word of the Lord, look, I turn to the fish, to further confound your disbelief".
And filled with the Lord's spirit, he began to preach to the fish, elaborating on their gifts given by God: how God had created them, how He was responsible for the purity of the water and how much freedom He had given them, and how they were able to eat without working.
The fish began to gather together to listen to this speech, lifting their heads above the water and looking at him attentively, with their mouths open. As long as it pleased the Saint to talk to them, they stayed there listening attentively, as if they could reason. Nor did their leave their place, until they had received his blessing.
He who had made the birds listen to the preaching of the most Holy Father Francis, gathered the fish together to listen to the preaching of his son, Anthony. (Rigaldina 9,24-28).