Soon after his return he joined two Jesuit fathers who were to visit a mission station of the Jesuits amongst the Onondagas (Iroquois) on a lakelet about thirty miles south-east of the present city of Rochester.  The Iroquois (whose language Radisson had learnt to speak) received them with apparent friendliness, and there they passed the winter.  But in the spring Radisson found out that the Onondaga Iroquois were intending to massacre the whole of the mission.  Instructed by him, the Jesuits pretended to have no suspicions of the coming attack, but all the while they were secretly building canoes at their fort.  As soon as they were ready for flight, and the sun of April had completely melted the ice in the River Oswego, the French missionaries invited the Onondagas to a great feast, no doubt making out that it was part of the Easter festivities sanctioned by the Church.  They pointed out to their guests that from religious motives as well as those of politeness it was essential that the whole of the food provided should be eaten, “nothing was to be left on the plate”.  They set before their savage guests an enormous banquet of maize puddings, roast pigs, roast ducks, game birds, and fish of many kinds, even terrapins, or freshwater turtles.  The Iroquois ate and ate until even their appetites were satisfied.  Then they began to cry off; but the missionaries politely insisted, and even told them that in failing to eat they were neglecting their religious duties.  To help them in this respect they played hymn and psalm tunes on musical instruments.  At last the Onondagas were gorged to repletion, and sank into a stertorous slumber at sunset.  Whilst they slept, the Jesuits, their converts, and Radisson got into the already prepared canoes and paddled quickly down the Oswego River far beyond pursuit.

Radisson next joined his brother-in-law, Medard Chouart, and after narrowly escaping massacre by the Iroquois (once more on the warpath along the Ottawa River) reached the northern part of Lake Huron, and Green Bay on the north-west of Lake Michigan.  From Green Bay they travelled up the Fox River and across a portage to the Wisconsin, which flows into the Mississippi.  Down this river they sped, meeting people of the great Siou confederation and Kri (Cree) Indians, these last an Algonkin nation roaming in the summertime as far north as Hudson’s Bay, until at length they reached the actual waters of the Mississippi, first of all white men.  Returning then to Lake Michigan, the shores of which seemed to them an earthly paradise with a climate finer than Italy, they journeyed northwards into Lake Huron, and thence north-westwards through the narrow passages of St. Mary’s River into Lake Superior.  The southern coast of Lake Superior was followed to its westernmost point, where they made a camp, and from which they explored during the winter (in snowshoes) the Wisconsin country and collected information regarding the Mississippi and its great western affluent the Missouri.  The Mississippi, they declared, led to Mexico, while the other great forked river in the far west was a pathway, perhaps, to the Southern Sea (Pacific).