WASHINGTON (AP) _ Angola's Marxist government, which is trying to end a rebellion backed by the United States, became the 152nd member of the World Bank and the International Monetary Fund on Tuesday and said it wants American investment.

''Angola is defending the West and we will give protection and a guarantee of security to American investors in our country,'' Finance Minister Augusto Teixeira de Matos told a news conference.

Despite U.S. support of the rebel Unita army headed by Jonas Savimbi, the Angolan government says it has protected U.S. oil installations with the help of Cuban troops.

The United States voted against Angolan membership but did not pressure other Western nations to join. It has the biggest voting power in both: 19.14 percent in the fund and 16.33 percent in the bank.

Both the bank and the fund have some members with Communist governments, including China, Poland, Hungary, Yugoslavia, Vietnam and Laos, but the Soviet Union is not among them and their total voting strength is small.

De Matos said investment in Angolan oil is expanding and predicted that American investment in it will increase. Investment in other kinds of business - fisheries, farming, transport and services would also be welcome, he added.