Aliens Act 1 of 1937 was a South African law aimed at curtailing Jewish immigration to South Africa just as it was increasing due to increased anti-Semitic repression in Nazi Germany. The Act instituted an Immigrants Selection Board which would screen every potential immigrant coming to South Africa from outside of the British Empire or Ireland with the power to either grant or withhold a permit to enter the country. One of the qualifications the Board considered when assessing immigrants was "assimilability", a term not defined by the legislation and thus left to the board to interpret subjectively. "Unassimilability" was a criticism made of the Jews and thus its use as a criterion was seen as a pretext for excluding Jewish migrants.
With the increase of Jewish immigration to South Africa in 1936, with the deterioration of the situation for Jews in Germany, so too increased political pressure within South Africa to curtail the migration. The United Party government led by Prime Minister Hertzog introduced the Bill in order to restrict and regulate immigration and exercise control over resident aliens.
The National Party's Daniel Malan, leader of the opposition, unsuccessfully attempted to amend the bill to explicitly exclude prohibit Jewish immigration, end the further naturalization of Jewish permanent residents of South Africa and close certain professions from Jews and "other non-assimilable races". Malan argued that Jews were getting the best jobs and that "the Afrikaner is suffering in consequence". Arguing for his amendment, Malan said of Jews, "Now the question arises with us, as a people, not only how we are going to keep them out in future, but how we are going to protect ourselves against those who are here."
The Jewish community within South Africa was divided on the bill. Jewish United Party MPs voted for it as it did not specifically mention Jews. Some members of the South African Jewish Board of Deputies supported the Bill as they feared Jewish immigration from Germany would intensify anti-Semitism within South Africa.
The Aliens Act became law in February 1937 and put into place an Immigrants Selection Board which was given the power to issue or refuse to grant permits to any prospective immigrant who was not a citizen by birth or descent of the British Empire or Irish Free State.
Jewish immigration from Germany fell as a result of the Act to a few thousand with approval for entry normally being given only to wives and young children of Jews already resident in South Africa or to their elderly parents or grandparents. In many cases, entry permits which had already been granted to relatives of South African Jews prior to the passage of the Act were cancelled and many German Jews who might otherwise had been able to find refuge in South Africa instead perished in the Holocaust.