Prime Minister Lionel Jospin's socialists came out of the elections on top, while President Jacques Chirac's conservatives suffered badly.
The caretaker leader of the Gaullist RPR, Nicolas Sarkozy, revealed he would be stepping down following the drop in right-wing support.
His party fell to 12.7% of the poll, compared to 25.6% in 1994; the RPR and its UDF allies won 22 seats, down from 28.
By contrast, the Socialist Party increased its share of the vote from 14.5% to 22.0% and its number of MEPs from 15 to 22.
The other big winners were the Greens, who won nine seats after drawing a blank in 1994, and the Hunting, Fishing, Nature, Tradition party (CPNT) which similarly won six.
The Green list was led by Daniel Cohn-Bendit, once known as "Danny the Red" for his leading role in the 1968 student uprising.
The party also credited its performance to worries about the effect of contaminated Belgian food.
The turnout was a record low for European elections in France, with only 45% having voted.