Foxes may attempt to mate with dogs if either animal is in heat and they are kept in captivity together, but foxes generally do not crossbreed with dogs in the wild. There have been reports through the centuries of dog-fox hybrids, but these are very rare and difficult to authenticate.
Foxes and dogs may look similar, but their behavior is quite different, making mating unlikely outside of artificial situations in captivity. For example, dogs are pack animals and go into heat at least twice a year, while foxes are solitary and only go into heat annually. Also, foxes are monogamous, so mating with a dog would require a fox that is both unpaired and has no potential partners of its own species. Finally, dogs have more than twice as many chromosomes as foxes, so there are genetic difficulties to producing any puppies from a mating. Scientific reports of fox-dog hybrids date to the 19th century and are mainly secondhand accounts with no genetic testing or examination of the purported hybrids.