Retailing magnate Robert Campeau's $6.6 billion takeover of the Federated Department Store empire topped a surge in Canadian investment south of the border.

The Campeau Corp. deal last month was the largest Canadian takeover in the United States. For Campeau, who left his native Sudbury, Ontario, with an eighth-grade education and retains a slight French-Canadian accent, it surpassed his $3.6 billion acquisition of New York-based Allied Stores Corp. in 1986.Canadian investment in the United States has encompassed prime real estate from Manhattan to Minneapolis, railways, pulp and paper mills, oil wells, telecommunications and many other industries.

Canada and the United States form the world's largest trading partnership, and a majority of the Fortune 500 companies long have operated above the 49th parallel.

But until recently, Canada had been relegated to junior partner. As recently as 1975, direct Canadian investment in the United States stood at just $5.4 billion.

By the end of 1986, however, this had soared to $29 billion, and the Canadian stake is now closer to $40 billion, according to Canadian government statistics.

The 1986 figure compares with $55 billion worth of direct U.S. investment in Canada, and the gap is closing.

"Over the past 10 years, Canadian investment in the U.S. is growing faster than U.S. investment in Canada," said business management professor Alan Rudman of the University of Toronto.