Dense hydrous magnesium silicate (DHMS) Phase A has been deformed at 11 GPa, 700 and 400 °C in the multianvil apparatus. Transmission electron microscopy (TEM) characterizations, using the weak-beam dark-field (WBDF) technique, have shown that at 700 °C, two different types of perfect dislocations are activated with ⅓〈2̄1̄10〉 and ⅓〈21̄1̄3〉 Burgers vectors. The ⅓〈2̄1̄10〉 dislocations glide in the basal plane, but for ⅓〈21̄1̄3〉 dislocations, no glide plane could be identified. At 400 °C, dissociation of ⅓〈21̄1̄0〉 dislocations is observed in the basal plane. Furthermore, prismatic and pyramidal glide planes were identified, with dissociations in pyramidal planes, at this temperature. Also a growth reticular merohedral twin has been observed; the twin law results from a rotation of 180° around the [15̄40] axis.