Tail tube assembly is an essential step in the assembly of long-tailed bacteriophages. Limited structural and biophysical information has impeded an understanding of assembly and stability of their long, flexible tail tubes. The hyperthermophilic phage P74-26 is particularly intriguing as it has the longest tail of any known virus (nearly 1 μm) and is the most stable known phage. Here, we present the structure of the P74-26 tail tube and introduce an in vitro system for studying the kinetics of tube assembly. Our high resolution cryo-EM structure provides insight into how the P74-26 phage achieves its flexibility and thermostability through assembly of flexible loops into neighboring rings through tight “ball-and-socket”-like interactions. Guided by this structure, and in combination with mutational, light scattering, and molecular dynamics simulations data, we propose a model for the assembly of conserved tube-like structures across phage and other entities possessing Tail Tube-like proteins. Our model proposes that formation of a full ring licenses the adoption of a tube elongation-competent conformation among the flexible loops and their corresponding sockets, which is further stabilized by an adjacent ring. Tail assembly is controlled by the cooperative interaction of dynamic intra- and inter-ring contacts. Given the structural conservation among tail tube proteins and tail-like structures, our model can explain the mechanism of high-fidelity assembly of long, stable tubes.