Gianaclis Caldwell suggests a 4mm knitting needle, as Kathrin said. I have been using just that. My first blue was not very blue inside, a lot less than what I had hoped, although I had only pierced once on the top and once on the bottom 7 days later, never re-opened holes and never did horizontal piercing. There was a great blue flavor, smell and the cheese was very much softened by the PR. With my second blue, which is still aging (about 45 days old now) I have pierced tope, bottom and sides 3 times now, still using that 4 mm needle, but considering using a chopstick-sized piercer. I have a young blue (about 10 days old) who also got its first piercing with the 4mm however when I crack open the older blue toma if it lacks in blue inside, I am going to up the diameter for the new guy.