In comments that had his audience chuckling at a conference on training for the priesthood, the 78-year-old pontiff revealed he is instinctively suspicious of overly pious candidates.
"I will tell you sincerely, I'm scared of rigid priests," he said. "I keep away from them. They bite!"
Francis resorted to humour to make a serious point that some people that are drawn to a clerical career are fundamentally unstable -- and that inevitably creates problems for the church if they are not weeded out.
"If you are sick, if you are neurotic, go and see a doctor, spiritual or physical," he added. "The doctor will give you pills. But, please, don't let the faithful pay for neurotic priests."
As well as assessing the spiritual state of candidates, seminaries should also seek to judge their physical and psychological condition, Francis argued.
"There are often young men who are psychologically unstable without knowing it and who look for strong structures to support them. For some it is the police or the army but for others it is the clergy," the pope added, warning that such disorders inevitably resurface at a later date.
"When a youngster is too rigid, too fundamentalist, I don't feel confident (about him). Behind it there is something he himself does not understand. Keep your eyes open!"
Improved selection and training of priests is a priority for the Church in the wake of the huge clerical sex abuse scandals which highlighted how easy it was for totally unsuitable candidates to be ordained.