Bacteriophage phi29 is one of the smallest and simplest known dsDNA phages, making it amenable to structural investigations. The three-dimensional structure of a fiberless, isometric variant has been determined to 7.9 A resolution by cryo-electron microscopy (cryo-EM), allowing the identification of alpha helices and beta sheets. Their arrangement indicates that the folds of the phi29 and bacteriophage HK97 capsid proteins are similar except for an additional immunoglobulin-like domain of the phi29 protein. An atomic model that incorporates these two domains fits well into the cryo-EM density of the T = 3, fiberless isometric phi29 particle, and cryo-EM structures of fibered isometric and fiberless prolate prohead phi29 particles at resolutions of 8.7 A and 12.7 A, respectively. Thus, phi29 joins the growing number of phages that utilize the HK97 capsid structure, suggesting that this protein fold may be as prevalent in capsids of dsDNA phages as the jelly roll fold is in eukaryotic viruses.