Rocky Mountain Sports is the official US Distributor of Tretorn Tennis Balls. The Tretorn Micro X has a consistent bounce long after other balls have gone flat so it's perfect for ball machines. Neither pressurized nor permanent pressure, Tretorn's micro cellular technology is patented worldwide and recognized by the ITF as the pioneer of new category of tennis ball. The result is a highly playable ball that does not lose performance over time. The Micro X ball is available in a box containing 72 solid yellow balls. Also available in two-tone yellow/white.