California became the first state in America to bar pet stores from selling dogs, cats and rabbits unless they come from animal shelters or rescue groups.

The law targets the controversial breeding facilities known as puppy mills or kitten factories, which often operate with little or no oversight and “house animals in overcrowded and unsanitary conditions without adequate food, water, socialization or veterinary care,” according to a fact sheet for the legislation, A.B. 485.

The new rules took effect after Gov. Jerry Brown signed them into law in October 2017, a gap meant to give pet store operators time to prepare their businesses for compliance.

While some cities and counties in California already restricted sales from unlicensed breeding facilities, the law was the first to apply across the state, the fact sheet said. Pet store owners who do not comply face a penalty of $500 per animal.

“Because pet stores are one step removed from the breeding of the animals they sell, store owners rarely know the breeding conditions of their animals,” the fact sheet said.

People may still buy dogs or cats directly from breeders.

The statewide legislation is the first of its kind to take effect in the United States, but other states are following suit with similar regulations that affect pet retail outlets.

Kevin O’Neill, vice president for state affairs of the American Society for the Prevention of Cruelty to Animals, said in an interview that California’s legislation appeared to be the beginning of a trend, with other statewide measures being drafted or considered in Washington State, New York and New Jersey.

“It did somewhat open up the possibility of moving it from a municipal effort to a statewide effort,” he said. “I think you are going to start seeing more and more states doing it.”

More than 250 cities and local governments across the country have similar measures that curtail mass breeding operations of dogs and cats, the A.S.P.C.A. said. In November, Atlanta became the ninth city in Georgia to prohibit so-called puppy and kitten mills. Instead, pet stores would exclusively offer cats and dogs for adoption, the City Council announced.

In Maryland, Gov. Larry Hogan signed a law in April that bars pet store operators from selling dogs and cats but allows them to host adoptions. Stores may benefit financially from the adoptions because they can lure potential customers, Mr. O’Neill said.

The law in Maryland, which takes effect in 2020, has not put a damper on business at Charm City Puppies in Columbia, which has sold about 95 dogs in the past month, said Ashley Lawson, an assistant manager. She said on Wednesday that the store sold only animals from federally licensed breeders across the United States.

“Today we are actually low on puppies because of the holiday season,” she said, although the store was selling a Cavalier King Charles spaniel for $2,499 and a Yorkshire terrier for $1,999.

“We are still looking forward to fighting it or overturning it,” she said, referring to the law. “It is all in the beginning stages.”

Some customers do not have the option to adopt, particularly if they are looking for a certain type of dog, she said. “That is why we believe they should be given a choice.”

In California, the law that took effect on Tuesday was written by two State Assembly members, Patrick O’Donnell and Matt Dababneh, both Democrats. The legislation’s fact sheet said the bill was meant to address “extremely minimal” federal standards, such as a requirement that says a cage may be only six inches larger than the animal it houses and may be cleaned just once a week.

There were also financial considerations, with approximately $250 million a year in taxpayer money used to house animals in local shelters, the fact sheet said.

Mike Bober, the president of the Pet Industry Joint Advisory Council, a national advocacy group, said in a telephone interview on Wednesday that the California legislation was “well intentioned but misguided” and that it could have a negative effect on about two dozen pet stores in the state.

Mr. Bober said it would have made more sense to institute a measure that provided better enforcement of federal law requiring stores to guarantee and post credentials from licensed breeders.

“People argue that a pet sale ban is going to be the magic bullet,” Mr. Bober said. “We think there are ways to do what the supporters of these bans want to do, without jeopardizing consumer protections and putting small businesses out of business.”

A dog at the Chico Municipal Airport in California, where animal agencies and nonprofit groups that helped rescue lost pets from the Camp Fire tried to reunite them with their owners. A new law in the state requires pet stores to sell only rescue animals.