Weetabix has moved the Alpen business without a pitch, following the agency's capture of the £12 million Weetabix account last month.
Creative duties for both the Weetabix and Alpen brands were previously handled by WCRS&Co but only the main Weetabix brand, including its Chocolate and Crispy Minis range, was included in the initial pitch.
BBH had won the main Weetabix business after a final pitch against Rainey Kelly Campbell Roalfe/Y&R and WCRS&Co in a process handled by Oystercatchers.
Alpen's last major ad campaign, launched in 2009, was called "we know you know how good it is". It featured a group of models exercising while eating bowls of Alpen.
Ben Fennell, the BBH chief executive, said: "It is great to pick up Alpen. It's another iconic brand and we're looking forward to telling its story."
Sally Abbott, the marketing director for Weetabix, said: "We're excited to be working with BBH on our stable of great brands and we're confident they'll deliver outstanding work for us."