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Eight new strains of deep‐sea hyperthermophilic sulfur reducers were isolated from hydrothermal vent fields at 9°50′N East Pacific Rise (EPR) and at the Cleft and CoAxial segments along the Juan de Fuca Ridge (JdFR). 16S rRNA gene sequence analysis showed that each strain belongs to the genus Thermococcus. Restriction fragment length polymorphism patterns of the 16S/23S rRNA intergenic spacer region revealed that these isolates fell into three groups: those from the EPR, those from fluid and rock sources on the JdFR, and those isolated from Paralvinellaspp. polychaete vent worms from the JdFR. The optimum‐temperature specific growth rates and the temperature ranges for growth were significantly higher and broader for those strains isolated from worms relative to those isolated from low‐temperature diffuse hydrothermal fluids. Furthermore, the worm‐derived isolates generally produced a larger array of proteases and amylases based on zymogram analyses. The zymogram patterns also changed with growth temperature suggesting that these organisms alter their lytic protein suites in response to changes in temperature. This study suggests that there is significant phenotypic diversity in Thermococcusthat is not apparent from their highly conserved 16S rRNA nucleotide sequences.