Walmart might not be the king of the discount stores after all. Dollar stores are becoming the go-to destination for low-income shoppers, a new study shows.

Marketing platform InMarket found that individuals from low-income groups were more likely to shop at Family Dollar and Dollar General than at Walmart. Family Dollar — which is owned by Dollar Tree — was the retailer that shoppers from low-income groups were most likely to visit, followed by Dollar General, Walmart, Dollar Tree, and Five Below, consecutively. Discount chain Fred's ranked sixth in the report.

For this report, "low income" was defined as a household bringing in $0 to $44,999 in income. The results were obtained by measuring location data from 50 million mobile devices.

Dollar stores like Dollar General and Dollar Tree are rapidly spreading across America. When it started growing, Dollar General stores were primarily located in rural and suburban areas. Today, the chain has more than 16,000 stores in 44 states and planned to open 975 new stores and remodel 1,000 existing stores in 2019.

As of August 3, 2019, Dollar Tree operated 15,115 stores in the US and Canada under the brands of Dollar Tree, Family Dollar, and Dollar Tree Canada. In contrast, Walmart has more than 4,700 stores in the US and has already closed at least 22 stores in the US and Canada this year.

Despite their growing ubiquity, dollar stores have come under scrutiny for their lack of fresh produce and for sometimes selling expired over-the-counter drugs. Messy stores and rat infestations have also plagued dollar stores across the country.

An Oregon Dollar Tree store was recently fined $1,900 for failing to deal with a rodent infestation that caused safety issues for workers.

Dollar General and Walmart declined to comment on the report. Dollar Tree did not immediately return a request for comment.