Thermal degradation of insulin complicates its delivery and use. Previous efforts to engineer ultra-stable analogs were confounded by prolonged cellular signaling in vivo, complicating mealtime therapy and of unclear safety. We therefore sought an ultra-stable analog whose potency and duration of action on intravenous bolus injection in diabetic rats are indistinguishable from wild-type (WT) insulin. Here, we describe the structure, function and stability of such an analog: a 57-residue single-chain insulin (SCI) with multiple acidic substitutions. Cell-based studies revealed native-like signaling properties with negligible mitogenic activity. Its crystal structure, determined as a novel zinc-free hexamer at 2.8 Å, revealed a native insulin fold with incomplete or absent electron density in the C domain; complementary NMR studies are described in a companion article. The stability of the analog (ΔGu 5.0(±0.1) kcal/mol at 25 °C) was greater than that of WT insulin (3.3(±0.1) kcal/mol). On gentle agitation the SCI retained full activity for >140 days at 45 °C and >48 hours at 75 °C. Whereas WT insulin forms large and heterogeneous aggregates above the standard 0.6 mM pharmaceutical strength, perturbing the pharmacokinetic properties of concentrated formulations, dynamic light scattering and size-exclusion chromatography revealed only limited SCI self-assembly and aggregation in the concentration range 1-7 mM. These findings indicate that marked resistance to thermal inactivation in vitro is compatible with native duration of activity in vivo. Such a combination of favorable biophysical and biological properties suggests that SCIs could provide a global therapeutic platform without a cold chain.