Structural　defects　of　high　density　have　been　observed　to　exist　in　the　as-cast　crystalline　approximants　Al-Ni-Rh　xi　and　Al-Pd-Fe　epsilon(16)　which　contain　columnar　atom　clusters　as　the　most　fundamental　building　blocks.　By　means　of　high-resolution　electron　microscopy　(HREM),　the　structural　and　distributional　features　of　these　defects　have　been　revealed.　It　is　clearly　shown　that,　accommodating　structural　defects　of　high　density　in　the　Al-Ni-Rh　xi-phase　can　lead　to　the　formation　of　a　unique　mixed　structure　in　which　nearly　regular　arrangement　of　structural　defects　can　be　established.　On　the　basis　of　HREM　observations,　long-range　structural　features　of　the　defective　xi-　and　epsilon(16)-structures　have　been　discussed　in　terms　of　the　characteristic　arrays　of　columnar　atom　clusters.