Walmart is once again America's grocery king, but rival Costco is rapidly gaining ground

It would take the four largest pure-play grocery chains combined — Kroger, Albertsons, Publix, and Ahold Delhaize — to finally outsize Walmart's grocery sales.

Frederic J. Brown/AFP via Getty Images

• More than one in five US grocery dollars is being spent at Walmart, for the third year in a row.
• At the same time, Costco is snapping up a growing share of grocery spending.
• The increases come as US shoppers look for more ways to get the most out of their household budgets.