The　spiral　and　ordinary　dislocations　both　running　nearly　perpendicular　to　the　(001)　growth　surface　of　thallium　acid　phthalate,　were　revealed　and　identified　by　x-ray　topography.　They　play　an　unexpectedly　important　role　on　that　face　by　providing　the　self-perpetuating　step　sources　so　as　to　accelerate　its　growth　rate　or　even　to　change　its　crystal　morphology.　©　1993　IOP　Publishing　Ltd.