The Girl Scouts of the USA accused the Boy Scouts of trying to recruit girls into their programs because of membership declines.
"I formally request that your organization stay focused on serving the 90 percent of American boys not currently participating in Boy Scouts ... and not consider expanding to recruit girls," wrote GSUSA President Kathy Hopinkah Hannan in a letter sent this week to the president of the Boy Scouts of America, Randall Stephenson.
Hopinkah Hannan and Stephenson met earlier this month to discuss the possibility of coordinating to boost membership, but the GSUSA president said she walked away feeling as though the Boy Scouts had already committed to expanding its programs.
BSA spokeswoman Effie Delimarkos said her organization was disheartened by the letter.
"We are disheartened to see the Girl Scouts pull away from the possibility of cooperation to help address the needs of today's busy families," she said Wednesday.
The Boy Scouts have had some coed programs since the 1950s, but the Girl Scouts said expanding those programs could be detrimental to their organization.
"Despite offering to engage in a constructive, collaborative sharing process, we were disappointed in the lack of transparency as we learned that you are surreptitiously testing the appeal of a girls' offering to millennial parents," Hopinkah Hannan said.
Delimarkos responded, saying that the Boy Scouts deeply respect the Girl Scouts.
The Girl Scouts, founded in 1912, and the Boy Scouts, founded in 1910, have faced sharp drops in membership in recent years.
The Associated Press contributed to this report.