The lineup change caused an online stir upon its announcement on Wednesday. Music blog Hipster Runoff broke the news, posting a photo of the band performing with a new drummer.
"Best Coast has and always will be Bobb [Bruno] and I," frontwoman Bethany Cosentino wrote. "No matter who has joined the band on tour, whatever lineup ensues, the band is simply the two of us. That said, these past two years have been full of nothing but the greatest of memorable adventures and I am so grateful that my friend Ali was a part of them when we were on tour."
On Wednesday, Koehler, previously of garage act Vivian Girls, tweeted about the news, but offered no reasons for the split.
Responding to a fan who asked, "What happened?," she wrote, "Your guess is as good as mine."
According to Cosentino, Koehler had been informed of the lineup change well in advance.
"One last thing: For those of you who are assuming things, Ali was informed 2 months ago that our live lineup was changing. There is no bad blood between any of us," she wrote.
For Koehler's part, she didn't seem too flustered by the news, spending the rest of Wednesday tweeting multiple times about snack foods and posting a link to solo material on her Bandcamp page.
"Whether it's in a basement or at an amphitheater, you will always see me playing music in some capacity. Hopefully sooner than later," she wrote.
Best Coast is currently working on a follow-up to 2010 breakthrough album "Crazy For You," recording with producer/composer Jon Brion in Los Angeles.
"As we enter into a new chapter and record for the band, we can't wait to share all of the new songs Bobb and I are working really hard on with all of you," Cosentino wrote on Facebook on Friday. "Thank you to all of our fans for supporting us. We are looking forward to experiencing our second record with you soon!"