LA JOLLA, Calif., March 10— Bob Crosby, a popular swing-era band leader who was a younger brother of the singer Bing Crosby, died on Tuesday at Scripps Memorial Torrey Pines Convalescent Hospital. He was 80 and had lived in La Jolla until moving to the nursing home two months ago.

His son Kris said the cause was cancer.

Mr. Crosby led big bands from 1935 to 1942, starting the Bob Cats, an eight-piece band-within-the-band, during that time. He sang but did not play an instrument.

His bands were known for swing and Dixieland sounds. The Bob Cats' hit records included "Big Noise From Winnetka," "South Rampart Street Parade" and "March of the Bobcats." Invited to Lead a Band

The youngest of five Crosby brothers, Bob Crosby was born in Spokane, Wash., and attended Gonzaga University in Washington State. He started his career with the Anson Weeks orchestra. In 1935, 11 New York musicians asked him to be their leader, and within three years the Bob Crosby Orchestra was playing in supper clubs and night spots across the country.

The Bob Cats then moved on to Hollywood, appearing in movies including "Let's Make Music," "As Thousands Cheer," "See Here, Private Hargrove" and "Pardon My Rhythm."

Although his brother Bing, who was nine years older, was not involved in the formation of the Crosby orchestra, it was often billed as the band of "Bing Crosby's brother."

During World War II, Mr. Crosby spent 18 months with the Marines, organizing and touring with bands throughout the Pacific.

Mr. Crosby disbanded the Bob Cats in 1942. But he reassembled them for a Dixieland concert in Los Angeles in 1951 and played with members of the group periodically over the next 40 years. He and some original members of his band played at concerts, balls and conventions until he fell ill last year, his son said.

In addition to his son Kris, he is survived by two other sons, Robert, of Los Angeles, and Stephen, of La Jolla; two daughters, Cathy Crosby of Los Angeles and Malia Donn of Dallas, and seven grandchildren.

George R Crosby