The structure of N-1, a virus infecting the filamentous, nitrogen-fixing blue-green alga Nostoc muscorem, has been further characterized. The edge-to-edge distance of the N-1 head is 614 ± 28 Å; the length of the tail is 1000 ± 62 Å. Flexible beaded fibers are attached to the N-1 neck. Optical diffraction of extended and contracted sheaths suggests that a rearrangement of protein subunits occurs upon contraction. The molecular weight of the viral DNA calculated from the sedimentation coefficient is 44 ± 3 × 106. Based upon the contour length of N-1 DNA molecules, the molecular weight is 41.8 ± 3.6 × 106. A survey has been made of the effects of a number of protein denaturing agents (urea and guanidine hydrochloride, anionic and cationic detergents, extremes of pH, and ultraviolet irradiation) upon the native viral morphology. For each agent tested, the first observable effect was to trigger a shortening (probably contraction) of the tail sheath. The most resistant viral substructure was the contracted sheath. From these investigations a hierarchy of increasing resistance to chemical degradation could be arranged: capsid, tail core, tail sheath.