The growth of a small crack controlling the high-cycle fatigue life of a precipitation-strengthened 6061-T6 aluminium alloy was critically investigated. As the applied stress lowered, the small crack was arrested for a long period (over 106 cycles) at grain boundaries before regrowth, which resulted in a significantly slow growth process. The morphological and crystallographic details of the small crack were then analyzed with focused ion beam and transmission electron microscopy. It was revealed that the small crack was formed along fine persistent slip bands (PSBs) whose structure was fairly different from that reported for other metals. The concept of PSB-limited fatigue strength may be extended to include the present material type.