A histological examination of a disorder known as “white knot” or “starch spot” was carried out on affected tubers of the potato cultivar Atlantic. Results revealed that the hard white lump located just below the periderm in affected tubers was comprised of cells containing a high number of starch granules, 2–4 times larger than those found in unaffected areas of the tuber. In the province of Ontario, “white knot” appears to be more prevalent in seasons when specific gravity is high.