Three new species of Herpetosiphon (unbranched, sheathed flexibacteria) are described, with their molar proportions of guanine plus cytosine (GC%). They were isolated from marine shores: H. persicus (GC% = 52) from Co. Galway, Ireland; H. cohaerens (GC% = 44) from Biarritz, France; and H. nigricans (GC% = 53) from Lagos, Nigeria. A fourth, isolated from orange tufts in a hotspring in Baja California, Mexico, is considered to represent a species hitherto described as a blue-green alga, Phormidium geysericola. Its transfer to the genus Herpetosiphon is recommended accordingly: H.geysericolus nov. comb. (GC% = 48).