A poll tax is a discrimination tax which was a pre-condition of the exercise of the right to vote. The practice of instituting a poll tax emerged in some US states in the late 19th century. After the right to vote was extended to all races by the enactment of the Fifteenth Amendment, many Southern states enacted poll tax laws which often included a grandfather clause that allowed any adult male whose father or grandfather had voted in a specific year prior to the abolition of slavery to vote without paying the tax. These laws achieved the desired effect of disfranchising African-American and Native American voters as well as poor whites who immigrated after the year specified.
The United States government did not levy poll taxes which blocked access to voting rights. Partly this is because the national government earned its revenues from income tax and excise taxes rather than from capitation, which required apportionment among the states. Also, this is because the national government did not conduct elections for its offices, instead delegating conduct of elections to the states.
The 24th Amendment, ratified in 1964, outlawed the use of this tax (or any other tax) as a pre-condition in voting in Federal elections. The 1966 Supreme Court case Harper v. Virginia Board of Elections extended this explicit enactment as a matter of judicial interpretation of a more general provision, ruling that the imposition of a poll tax in state elections violated the Equal Protection Clause of the 14th Amendment to the United States Constitution. In a two-month period in the spring of 1966, the last four states to still charge a poll tax laws had those laws declared unconstitutional by Federal courts, starting with Texas on February 9. Decisions followed for Alabama (Mar. 3) and Virginia (Mar. 25). Mississippi's $2.00 poll tax was the last to fall, declared unconstitutional on April 8, 1966, by a Federal panel in Jackson, Miss. 
- US Treasury Department : Education : Fact Sheets : Taxes, History of the U.S. Tax System
- The World Almanac 1966, p. 68