Levels of DNA strand breakage were measured, using the comet assay, in cells from vent mussels, Bathymodiolus azoricus, from three contrasting vent fields on the mid Atlantic Ridge. Different levels of DNA damage were recorded in untreated mussels, shortly after collection, and it was animals from the shallowest, and less active, Menez Gwen vent field (840-m depth), which showed the greatest amount of damage. In contrast to animals from two deeper and putatively more toxic sites, Menez Gwen animals went on to repair this damage and were able to survive under laboratory conditions at 1 bar pressure for several months. Animals from the two deeper sites showed both higher levels of initial mortality and a much reduced capacity for survival at 1 bar. The differences in DNA damage levels at the time of collection were interpreted as an expression of differences in cell viability/enzyme activity rather than a reflection of any differences in their natural environmental conditions. Small B. azoricus showed a capacity to repair DNA damage, whereas this ability appeared to be lacking in large individuals. By reproducing at a relatively early age, the deep-sea vent fauna may be able to resist the toxic effects of its environment by exploiting this natural, stage specific capacity to repair damaged DNA.