Asimov's father: Citizenship

pg 80 of In Memory Yet Green: The Autobiography of Isaac Asimov, 1920-1955

"My father came to the United States with every intention of making it his permanent home, and that meant citizenship. He took out his "first papers" a declaration of intent, after he had been in the country for three years, and in September 1928, when he had been in the United States for 5 1/2 years, he received his second papers, the real thing, and became a naturalized citizen  at the age of 31."

His mother wouldn't become a naturalized citizen until 10 years later, in 1938.