A Yorkshire Terrier suffered a broken leg after being bitten by another dog in Brimington.
The incident happened on Thistle Park, off Station Road, at around noon on Thursday, March 24.
Police say the Yorkshire Terrier also suffered severe bleeding in the attack but is now recovering after extensive treatment at the vets.
The attacking dog was with a white man, who was aged between 40-45 years old.
Officers say the dog is described as large, cream, black and grey and part terrier. It had a large head.
Anyone with information should contact PC 2738 Stuart Hind on 101, quoting incident number 216/25/03/16, or send him a message online by visiting the Contact Us section of the Derbyshire police website.
Alternatively, contact Crimestoppers, anonymously, on 0800 555 111.