Theriodopteryx ephemeraeformis commonly known as bag worms produce ultrafine silk fibers that are remarkably different than the common domesticated (Bombyx mori) and wild (Saturniidae) silk fibers. Bag worms are considered as pests and commonly infect trees and shrubs. Although it has been known that the cocoons (bags) produced by bag worms are composed of silk, the structure and properties of the silk fibers in the bag worm cocoons have not been studied. In this research, the composition, morphology, physical structure, thermal stability, and tensile properties of silk fibers produced by bag worms were studied. Bag worm silk fibers have considerably different amino acid contents from those of the common silks. The physical structure of the bag worm silk fibers is also considerably different compared with B. mori and common wild silk fibers. Bag worm's silk fibers have lower tensile strength (3.2 g/denier) and Young's modulus (45 g/denier) but similar breaking elongation (15.3%) compared with B. mori silk. However, the tensile strength and Young's modulus of bag worm fibers are similar to those of the common Saturniidae wild silk fibers. Bag worm silk fibers could be useful for some of the applications currently using the B. mori and wild silk fibers.