Swiss voters are due to go to the polls to decide on measures aimed at severely restricting immigration.
The ballot marks the second time this year that non-EU country Switzerland has held a referendum on immigration.
Last February voters narrowly backed reintroduction of quotas, effectively opting out of a free movement of people agreement with Brussels.
The new proposal, known as Ecopop, goes much further, limiting net immigration to just 0.2% of the overall population.
Switzerland's population is just 8.2 million - but that is still more than a million more than it was 20 years ago.
The BBC's Imogen Foulkes in Geneva says that, while unemployment is low and living standards are high, many Swiss worry about overcrowding and environmental degradation.
Supporters of the Ecopop initiative say restricting immigration will safeguard Switzerland's environment by reducing the need for new transport links and new housing.
Ecopop also aims to limit overpopulation abroad, by devoting 10% of Switzerland's overseas aid to family planning in developing countries.
Opponents, among them all the major political parties, say the proposals will be bad for the economy - 25% of the Swiss workforce is foreign, and business leaders want to be able to recruit skilled labour from across Europe.
Meanwhile, many environmental groups argue that if the Swiss really want to protect their environment, they should start by adjusting their own lifestyles.
Opinion polls show a close vote, with latest indications suggesting that while many Swiss are worried about immigration, the Ecopop measures may be a step too far.