Willow is a serious chewer, she's a small Retriever mix and she's 9 months old.  These balls lasted much longer than the regular balls you'd find at the pet store.  She loves to peel the felt covering from the balls.  It seems like she chews them for a while to soften them up and then she begins to pull and peel the covering.  Then the regular balls split in half along their seam.  These balls are much harder to peel the felt off.  It took her over a month to get the skin off of the ball and when she finally did it didn't split in half.  The blue rubber stripe that swirls around the ball makes it a very sturdy ball.  I highly recommend these tennis balls.