Uni-ball Signo Bit UM-201 Gel Ink Pen Refill - 0.18 mm - Black

Refills also fit Zebra Sarasa Stick and Sarasa Clip retractable pen bodies, if you gently remove the small plastic thing at the end of the refill. Unfortunately I've had two Signo Bit pen bodies crack where the body screws together. I could not fix it with glue or tape, but the refill was brand-new (and I luv the Bit!) so I tried to fit it in many different old pen bodies. I did notice the ink dry&scratchy when I used a Sarasa Clip body because this type of pen is meant to be protected by a cap when not in use, but the Clip is retractable, no cap.

YMMV, good luck :)

This pen is so fine and the ink lasts a long time. I've ordered several and friends all love it. You have to have a light touch, though, and not press too hard!