CREATE TABLE table_72726 ("Series #" FLOAT, "Season #" FLOAT, "Title" TEXT, "Directed by:" TEXT, "Written by:" TEXT, "Original air date" TEXT)

Which Season originally aired on September 17, 1955
SELECT "Season #" FROM table_72726 WHERE "Original air date" = 'September 17, 1955'
SELECT "Season #" FROM "table_72726" WHERE "Original air date" = 'September 17, 1955'