The invention provides a novel .beta..sub.2 adrenergic receptor agonist in crystalline salt form. The invention also provides pharmaceutical compositions comprising the novel .beta..sub.2 adrenergic receptor agonist in crystalline salt form, formulations containing the pharmaceutical compositions, methods of using the crystalline salt to treat diseases associated with .beta..sub.2 adrenergic receptor activity, and processes useful for preparing such crystalline compounds.