The Quebec Liberals easily hung onto Tony Tomassi's former seat of Lafontaine in northeast Montreal in Monday's byelection, but lost Argenteuil to the Parti Québécois by 501 votes — the first time the provincial riding has slipped from the Liberals' grasp in 46 years.

In Lafontaine, Quebec Liberal party president Marc Tanguay won the seat back for the Charest government easily, with 53.32 per cent of the vote.

Opposition parties had hoped to capitalize on the controversy surrounding Tomassi, the one-time family minister who was kicked out of the Liberal caucus in May 2010 over allegations he had accepted rewards from a company that received government money.

Tomassi — who sat as an independent until resigning from his seat last month — is now facing charges of fraud and breach of trust.

However, both the Parti Québécois and François Legault's fledgling Coalition Avenir Québec straggled far behind the Liberal's Tanguay, fighting for a distant second place.

The PQ's Frédéric St-Jean garnered 17 per cent of the vote, compared to 15.58 per cent for the CAQ's Domenico Cavaliere.

A tight race in Argenteuil

In Argenteuil, the race remained too close to call most of the evening, as the lead moved back and forth between Liberal candidate Lise Proulx and her Parti Québécois opponent, Roland Richer.

Richer was declared the winner when all 180 polling stations reported at around 11 p.m., taking 36.16 per cent of the vote to Proulx's 33.40 per cent.

Que. premier Jean Charest consoles Liberal candidate Lise Proulx after her defeat in Argenteuil on Monday evening.

Former Bloc Québécois MP Mario Laframboise, running for Coalition Avenir Québec, placed a distant third, with 21.40 per cent of the vote.

The provincial leader of Quebec's Parti Vert, Claude Sabourin, took just 2.99 per cent, a fraction of a point ahead of Yvan Zanetti of Québec Solidaire.

Even when she was in the lead, early in the evening, Proulx lagged behind the percentage of the vote count her predecessor in the riding, David Whissell, received in the 2008 general election.

Whissell gave up his Argenteuil seat in December 2011, 27 months after he resigned from cabinet over a controversy about his stake in a construction firm.

Premier Jean Charest expressed little doubt Proulx would get another chance to try her hand at provincial politics.

"What I retain from this byelection race in Argenteuil is the discovery of an extraordinary woman who conducted a magnificent campaign," Charest said, Proulx at his side at her campaign headquarters in Lachute, Que. "I predict she will be the next member for the riding of Argenteuil."