Some of the larger Cub Scout packs will be able to choose to welcome both genders.
Michael Surbaugh, the BSA's chief scout executive, said reinforcing positive character traits is important for everyone.
As for older girls, the organization says it will announce a program next year that will enable them to earn the highest rank of Eagle Scout.
In another pre-emptive move, the Scouts will soon allow girls to become Cub Scouts and work their way up to Eagle Scout, the group's highest rank, after an unanimous vote by the scouting board of directors.
"The values of Scouting - trustworthy, loyal, helpful, kind, fearless and reverent, for example - are important for both young men and women", he said.
The board noted that all Cub Scout dens "will be single-gender - all boys or all girls", however.
"I formally request that your organization stay focused on serving the 90 percent of American boys not now participating in Boy Scouts. and not consider expanding to recruit girls", wrote GSUSA President Kathy Hopinkah Hannan in a letter to the BSA's president, AT&T Chairman Randall Stephenson. For some families, scouting programs that welcome both boys and girls could be a welcome convenience. Over the years, the BSA has opened its doors to openly gay scouts (2013), then openly gay scout leaders (2015), then transgender boys (2017), and now girls.
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In a statement, the group said that after "years of receiving requests from families and girls", it "evaluated the results of numerous research efforts" and came to its decision. That didn't sit well with the Girl Scouts of the U.S., whose leadership accused the Boy Scouts of running a "covert campaign to recruit girls".
In August, the president of the Girl Scouts accused the Boy Scouts of seeking to covertly recruit girls into their programs while disparaging the Girl Scouts' operations.
BSA at the time pushed back against such claims, explaining they'd always been discussing introducing girls into the program.
"I just want to do what the Boy Scouts do - earn the merit badges and earn the Eagle Award", she told NBC News.
The units, however, will still be single-gender.