Park County Health Officials have tested another 179 National Park Service employees for COVID-19.  All of these tests came back negative, according to a Yellowstone National Park press release. This is the second round of negative tests. A little over a week ago 43 were tested and all of them also came back negative.

Routine surveillance testing will continue for COVID-19 throughout the season for first responders and those who work directly with the public as part of the park's reopening plan.
The park also sampled wastewater systems in Gardiner, Mammoth Hot Springs, and Old Faithful with all tests coming back negative for COVID-19. Samples were taken May 18 to 26th.
Superintendent Cam Sholly stated:
I want to thank the states of Wyoming and Montana and our county health officials for partnering with us to increase surveillance testing capacity. This proactive testing is the only way for us to detect cases early and respond quickly to any employees who have the virus.

Yellowstone National Park Photos