The Constitution Alteration (Senate Casual Vacancies) Bill 1977, was a successful proposal to alter the Australian Constitution concerning the filling of casual vacancies in the Senate. It was put to voters for approval in a referendum held on 21 May 1977. After being approved in the referendum, it received the royal assent and became law on 29 July 1977.

Prior to the amendment:

• the legislature of the relevant state was not required to have regard to the political allegiances of the replacement senator, and
• the new senator's term continued only until the next general election for either the House of Representatives or the Senate, or the end of the original senator's term, whichever happened earlier.

The amendment changed this procedure by providing that:

• a state legislature replace a senator with a member of the same political party, and
• the new senator's term continue until the end of the original senator's term.

Its intended purpose was to prevent major changes in the balance of power in the senate in the middle of a parliamentary term, but as it did not provide any time limit within which the appointment had to be made, the state legislature remained free to decline to fill the vacancy. As Section 11 of the Constitution permits the Senate to carry on despite the failure to fill any vacancy, the amendment did not completely solve the problem.

On the same day as the vote on the casual vacancies amendment, four other questions were also put to the electorate. These concerned the:

• Simultaneous elections amendment
• Referendums amendment
• Retirement of judges amendment
• National song plebiscite.