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.
I have to question whether this bill is really about restricting Muslims from entering the country. Having watched what is happening in Denmark and France, restricting Muslim immigration would be a very wise thing to do. At 0.2%, with most immigrants coming from Italy, Germany and Yugoslavia, it leaves little room for Muslims.
|Switzerland recruited high numbers of foreign workers in the 1950s and 1960s|
EU citizens make up the vast majority of immigrants in Switzerland
The largest group of foreign nationals living in Switzerland is from Italy; immigration from Italy started more than a century ago, but difficulties getting Swiss nationality meant many families remained Italian
The second largest group comes from Germany, and the third largest comes from the former Yugoslavia