Achilles Catsonis, 79, a retired Washington lawyer who was a former adviser to the Greek Embassy and was long active in Greco-American relations, died Friday at Holy Cross Hospital. He had suffered from a heart ailment.

Mr. Catsonis moved to Washington in 1929 and was admitted to the Washington bar. He retired in the mid-1970s.

In addition to his law practice he devoted much time to fostering American-Hellenic relations through the efforts of the Order of AHEPA (American Hellenic Educational and Progressive Association).

Mr. Catsonis served as supreme secretary of AHEPA from 1927 to 1934, and as supreme president of the organization from 1934 through 1935. He also served as editor-in-chief of AHEPA Magazine.

During the late 1920s and 1930s, he traveled to all 48 states and spoke about Greek politics, culture, and the need for Greco-American friendship.

Mr. Catsonis served as an adviser to the Greek Embassy in Washington during World War II, acting as a spokesman for the Greek cause in Washington.

Speaking before the Washington Club in November, 1943, he pointed out that the Greek defeat of the Italian invasion "gave to the Allies their first victory in a long series of defeats, and to the occupied countries, an encouraging omen of brighter days to come."

Although Greece was then overrun by German forces, Mr. Catsonis interpreted even this as a victory of sorts,". . .for when time was of the essence, Greece stole time from Hitler and gave it to the Allies . . . Greece set back Hitler's timetable on all fronts, forced him to open a second front in the Balkans, delayed the attack on Russia by two months."

Many historians today agree that it was Hitler's Balkan invasion in the early spring of 1941 that helped doom his June invasion of the Soviet Union. After early impressive victories against the Red Army, Hitler was slowed by a terrible Russian winter in 1941-1942, which gave the Soviets a chance to recover.

Mr. Catsonis also served for a time as regional director of the Greek War Relief Association and secretary of United Nations War Relief, Inc., in Washington.

In 1949, he was awarded the Order of Phoenix with the Gold Cross by King George II of Greece for his wartime services.

Mr. Catsonis served for several years in the early 1960s as a member of the Washington Public Welfare Advisory Council.

A native of Lemnos, Grece, he came to this country at the age of 11. He earned a bachelor's degree at Hamilton College, a master's and two law degree at Syracuse University, and a doctorate in political science at Georgetown University.

Mr. Catsonis was a member of Saint Sophia Greek Orthodox Cathedral in Washington.

He is survived by two daughters, Electra C. Beahler, of Santo Domingo, Dominican Republic, and Aliki M. Nearing, of New Carrollton, and two grandchildren.