Wal-Mart is quietly growing its online grocery pilot, rolling out the service in two more urban markets this week.

If you live in Dallas or Orlando, Florida, you will now be able to order your groceries on Walmart.com and have them delivered to your house via Uber, the big-box retailer announced Monday.

Until now, the pilot service with Uber had been available in Phoenix and Tampa, Florida. Wal-Mart has also been running its own grocery delivery service in Denver and San Jose, California.

"We've been testing delivery in a number of ways for a while now in key markets across the country," Mike Turner, Wal-Mart's vice president of e-commerce operations, said in a statement.

"In some areas, we're trying general merchandise deliveries led by associates. In others, we're testing grocery delivery using Walmart trucks and drivers."

Just last year, Wal-Mart began testing grocery delivery through services such as Uber. The company also has been testing delivery by its employees, who drop purchases off at customers' homes after a day at work.

In its latest earnings report, Wal-Mart management highlighted online grocery as a bright and booming business. CFO Brett Biggs told CNBC last week that the retailer is pleased with what it's been doing in food, and "online grocery continues to grow rapidly."

During the second quarter, food categories saw their strongest quarterly comparable sales performance in five years, Wal-Mart said. And digital sales were up 60 percent for the period, fueled in part by the retailer's acquisition of Jet.com last September.

More and more retailers and grocers alike are looking to grab a share of the online food market before it's too late.

Amazon's AmazonFresh program has been a pioneer, prompting other grocers to consider beefing up their e-commerce business or risk losing sales from consumers who are increasingly in search of quick and convenient options.

Instacart has been another big player. The same-day grocery delivery start-up has been growing its list of partnerships with retailers including Whole Foods, Costco, CVS and recently Aldi.

Target is also gradually making moves to speed its delivery to shoppers' homes, but the big-box retailer's food business isn't as developed as that of some of its peers.

Later this week, shareholders of Whole Foods are set to vote on Amazon's takeover bid. While investors are expected to approve the offer, there are many unknowns regarding how this deal, once completed, might impact the supermarket industry.

"The message for all retailers, especially grocers, is simple: Engage with your customers in the manner they want and expect, whether in the digital realm or through traditional channels, or be prepared to lose market share to those that do," said Sam Cinquegrani, CEO of ObjectWave.